There is a simple in flight test used with RC gliders.

Some of which are getting to be quite big these days and a subsection of
which fly faster than 500 mph (up hill, into wind, that was measured at)
So, probably, has some application in our sport.

Fly at whatever speed you are normally trimmed at and put the glider
into a dive (not particularly steep, say 30 deg nose down) and let the
glider do what it wants from there. (staying below Va or Vne depending
on conditions)

If the glider stays at that angle an RC glider has it's CofG perfect (for
us
that means our CofG is a bit too far back as we need more stability in
pitch than the RC gliders)

If the glider pitches down and increases the dive, your CofG is too far
back (flying surfaces are keeping the nose down and their effect
becomes more pronounced as speed builds)

If the glider recovers back to flat n level then the CofG on ours is about

right, on an RC glider it's a bit too far forward.

The other check to make before you do this: note where your trimmer is
in normal flight, after landing put the trimmer back there and look at the

elevator. Ideally, when your CofG is right, your elevator should be in line

with the horizontal stab at "Normal" trim (it probably isn't as your CofG
will be a bit forward to give the glider longitudinal stability)

So, the RC glider test will show where your inflight CofG is (note, your
center of pressure will move relative to your CofG depending on speed)
Modern gliders are designed to be stable.
Older gliders too (but usually not so well)

A slightly forward CofG gives you stability
A very forward CofG gives you lots of drag.
A rearward CofG gives your wife hope for the next husband to be better .