The standard lapse rate is 3.5F per 1000'. So it you are going to fly
at 5000' above the ground, the temp up there will be 17.5 degrees
colder. Giving 5 degrees for variation, that is 22 degrees colder. So
in this case it would have to be 44 degrees or higher on the ground to
not have ice 5000' higher. (I've done some rounding). Of course you
COULD have inversion. But inversion means it is warmer above. You could
also have a non-standard lapse rate. But this is a calculation worth
doing.

Usually people have a place on the airplane where ice forms first. It
will usually be something thin, like an antennae or strut. You can use
that as a guage. Thin frontal surfaces ice up first and worse. Some
sort of physics thing about the air in front deflecting the moisture.
More deflection with fatter curves. But usually my first indication of
ice has been decreased aircraft performance (if I am in level flight).
Descending or climbing is a little harder to tell.

As for decending. if it is rime, I think 1500' of undercast is usually
fairly safe. If it is freezing rain then no-go. Someone mentioned 1500'
per minute. But the most important thing is keeping the airplane
upright and not overspeed. You may want to use your standard decent
rate and not go around doing something different. The word is that
horizontal stablizer icing is the worst thing on decent. Don't use
flaps for landing if you have ice when you land and land a little
faster. Do everything at higher airspeed if you have ice. This will
keep you above stall speed (hopefully). See that is the problem. Due to
different shape of the wing due to ice the plane will stall at a higher
airspeed.

Another item. We all know to stay out of thunderstorms, but you should
stay out of towering cumulus too, even if they aren't thundering and
lighting. It is icy in there. I think the abreviation for these is CB.
Don't fly in CB's.

A lot of times there is just a thin layer of clouds, like 1000' thick
that you have to climb and decend through. And there is ice in them.
That one is a close call. If you have a PIREP or other good info and
know it is clear on top, it is tempting to try and climb on up. The
lower the ceiling is below, the riskier this is. You will have to
evaluate and make your own judgement. Talking to a pilot who has just
landed is very valuable. Hang out at the fuel desk. Maybe someone will
come in who just flew through it. Sometimes airmets are wrong. Ice
tends to be worse at the initial part of the front, and an old icing
airmet MAY be invalid, just hasn't been cancelled yet. All depends. But
if someone comes down through it and he says he didn't pick up any ice
and he doesnt think anyone would, that can be helpful information.

Like someone else mentioned. you can venture fairly safely into icy
clouds if you have warm VFR conditions under you, above the MEA, no
mountains and flat terrain. If you ice up, just descend.

If you tell ATC you have ice, be prepared to answer their questions
about it. Rime or clean? What is the temperature etc. They usually want
a full report. After all, if you have ice, YOU'RE the expert.

Having a turbocharged and high power to weight ratio is a godsend in
ice. Just climb up through it to clear above. Thats what the airlines
do. Sometimes they dont even turn on their deice cause it takes away
power. They'd just rather climb like a homesick angel and get out of
it. All depends. But the point is, being able to CLIMB is a godsend.
They need their deice mostly on decent. If you cant outclimb it, and
you have low ceilings below you, my suggestion would be TURN AROUND,
unless you know that you have VMC in front of you SOON. Presumbably,
you didn't get ice back there and if you go back there you wont
accumulate it. Whatever you do , don't just try and hold altitude while
you go slower and slower. That's how you stall and spin. Even a decent
below the MEA is safer than that strategy. And ATC will be telling you
he cant give you any lower. Just declare an emergency if you have to.
MEA's have at least 1000' cushion, usually more. It would be a
desperate crap shoot to be accumulating ice, unable to hold altitude,
and have to decend below the MEA. Good way to run into something. This
is the reason why I don't do this kind of stuff.

Like I said, we all get ice now and then if we fly in the clouds below
freezing. But don't get yourself boxed in. If you don't have an out,
fly to where you do have an out. And don't fly into a trap where you
can't outclimb the ice, and you can't decend because of low ceilings.
That is where the icing accidents occur. That and on landing with a
load of ice.

I hope that helps. Bottom line, you can't really do much about ice
except stay out of it, or get out of it if you are in it.