A few additions to Kirk's excellent points - from the background of former
fighter pilot, current airline pilot, and current glider pilot:

1. We often surprise each other in sailplanes with how hard it is to see
each other. Don't expect an airline pilot to be any better at it! The
fighter pilot at least will have good visual acuity and is used to looking
for small targets.

2. Airline pilots don't carry sectionals - at the speeds we operate, there
would be little time to use them anyway. Fighter pilots will carry a low
level map and will have thought about visual traffic conflicts, wires,
terrain, etc in the planning stages. At the speeds they operate, they
aren't looking at those maps very often, once airborne.

3. The busier glider operations are notam'd and often referred to by atc
controllers. If you have an operable transponder, you will *normally* be
called out by atc and if TCAS equipped, airline pilots will be aware of your
location. They would still have to see you to maneuver away from you. (See
note 1.) Big airliners are not very maneuverable (mine - the Boeing 737 -
is limited to 2.5 g!).

4. Fighters are a different case. They don't have TCAS and only some of
them have the ability to interrogate/detect transponder targets. Some of
them have air intercept radar capability, but sailplanes are small radar
targets and will often (usually!) be filtered out because of their low
speeds and altitudes - like highway traffic. If they are at low altitude,
fighters usually operate at high speed (420 - 540 indicated, except the
A-10). As Kirk pointed out they will almost never be alone, but will be in
formations of 2 - 4. When low level (100 to 1500 agl, most commonly 300 -
500agl), they will normally *not* be receiving traffic information from ATC.
When operating in a MOA, there may be intercept controllers who can call out
glider traffic, but again, without a transponder, it is unlikely. The
formations will vary, but most pairs of flight lead and wingman will be
laterally spread by 5000 to 10000 feet, for visual lookout. The flight lead
will be spending quite a bit of his time looking forward for threat
detection and navigation, but the wingman will be spending less time looking
forward because he must maintain formation. If they see you, they have an
excellent capability to avoid you. Head on and tail on, the sailplane has
the tiny visual profile that fighter designers dream of.... In other words,
you are nearly invisible unless you have a wing up in a turn/thermal.

5. As Kirk said, the primary threat is at 6 o'clock, because it is the
hardest to see - essentially, only the overtaking aircraft has a reasonable
chance of avoiding a collision. Therefore, if you know you are operating in
a high threat area: MOA, low level route, approach corridor, VFR flyway,
near an airport etc, I would "belly check" periodically, depending on the
nature of the threat. The timing is based on the amount of time it takes for
the threat aircraft to close from outside visual range to hitting me from
the 6 o'clock position. I use visual ranges of 8nm for airliners, 5 nm for
small commercial jets (corporate and regional jets) and fighters, and 3 nm
for light aircraft - adjust as your visual acuity and experience dictate. I
use worst-case speeds as follows: airliner and small jets - 4 nm/min,
fighters - 8 nm/min, and light aircraft - 2.5 nm/min. Combing detection
ranges and times, I calculate: airliners - 2 min, small jets - 1 min and 15
sec, fighters - roughly 40 sec, and light planes - approx 1 min and 15 sec.
So... if you are straight and level for more than these times, there is
sufficient time for an aircraft to move from outside (my) visual range to
the same airspace as my (your) little pink body. As you would probably
guess, fighters are the worst case because of their relatively small size
and high closure rate. On the positive side, there are typically more
eyeballs with better acuity and better maneuverability involved.
Interestingly, small jets and light aircraft are not that far behind, as far
as detection time is concerned. In my experience they are far less likely
to see you than the fighters. The same is true for airliners, but because
of their size you have more time to see them coming...

6. How to do a belly check: No, I don't hack a stopwatch, but I keep the
above times in mind with respect to the likely threat for my area. My
primary threat is small jet/light aircraft that operate on various
highway/flyways and approach corridors. Away from these specific areas,
traffic density is extremely low. First clear your "new six" - if you are
going to turn left, look to the area behind to the right 4 - 5 oclock
position - this will be your new blind spot. Next clear your new nose
position - this is where you are going to roll out. Finally make a 45 deg
turn to the left and visually clear your "old six", which is now at your
left 7 to 8 o'clock. Often/usually, a belly check can be incorporated into
turns you are going to make anyway, for other reasons. When you visually
clear, make sure you focus on something on the horizon, otherwise you are
only visually clearing out to an arms length. If I really need to hold a
straight line, I do the belly check as a gentle 45 deg turn to each side.

7. In a thermal, periodically check to the outside of your term to clear
your "new six". If there are other sailplanes with you in the thermal, of
course they are the primary threats for midair, but you still need to check
for other aircraft. Fortunately, you are easier to see while turning - as
long as the other pilots are looking...

8. Proximity to clouds. You need to think about what you are doing when
you are near cloudbase, in proximity to likely IFR traffic. If you are 500'
below cloudbase (perfectly legal), and an airliner descends out of the cloud
at 250kt on his descent profile on collision course (perfectly legal), there
may be as little as 20 seconds to impact. If you are tail on when this
happens - good luck. I'm sure no one would ever be right at cloudbase on a
nice day, because that would violate the FARs - more importantly, you are
"rolling the bones" every time you do this on a known approach corridor.

9. Conclusion. If you fly in a high airliner/small jet threat area and
can afford a transponder it will help other people see/avoid you. If your
primary threat comes from military operations in MOAs, I would not spend the
money on a transponder unless I knew those fighters have intercept/atc
controllers passing them information. The various TPAS - type devices will
help your see/avoid efforts and should help in the case of fighters,
although the flight lead is likely the only one squawking in the formation.
Only you/your club knows the primary threats for your particular operating
area and you need to understand what they are. Taylor your altitude
awareness/cloud avoidance and belly check frequency to the nature of your
local area. Don't cede visual lookout/avoidance responsibility to someone
else - ever. Sailplane right-of-way is a myth in most situations and a
comfort only to your survivors/legal counsel.

Hope this helps.

Glen