How Can I Determine Gearing In My 94 Stang Gt?

Jack up the rear of the car.
Draw a line on the drive shaft lengthwise.
Immobilize one wheel so that it can't turn (skip this part if you have a working posi).
Rotate the other wheel one full turn while observing the mark on the drive shaft.

If the drives shaft rotates three and a half times, you have 3:55 gears,
three and three quarters = 3:73
two and three quarters = 2:73
etc.

If you jack up the back of the car and rotate the wheel forward and the opposite wheel rotates in the same direction, you have a working posi.
If you rotate the wheel forward and opposite rotates backward, then you have a non-working posi or none at all.