Aft c.g.s can be tested by diving... if the plane pulls out from a moderate dive, after being trimmed for level flight,
and pushed over, the c.g. can go back a bit.
If the plane doesn't recover, or the dive increases, the c.g. is at the aft limit.
Note this test is speed sensitive... as speed increases in the dive, the plane can increase the dive angle.. another indication the c.g. is at the aft limit.