Jack the rear of the car up and support it on jackstands. Mark the driveshaft with a piece of chalk. Put the trans in neutral.
Rotate (or have a buddy do it) the rear wheels 10 times and count the number of times the driveshaft turns.
If it turns nearly 33 times, you're 3.27:1 If it turns 35-1/2 times, 3.55:1. ~37 times is 3.73:1 and 41 times is 4.10.
If the speedo is off, you can also do some math. Find a car with an accurate speedo. Follow him at, say, 70MPH and note your own speedo reading. Assuming your speedo is calibrated for 3.27s, the error in the speed reading can be used as a multiplier to get the error in the gear ratio. For instance, if your speedo read 80 while the car was ding 70 (as indicated by the other guy's speedo reading 70), then your error is 80/70 or ~1.14. Take the stock 3.27 gear that the speedo is calibrated for and multiply by 1.14 to get the rear gear. In this case, 3.27x1.14 is ~3.727 giving a rear gear of 3.73:1.
EDIT: FWIW, if you're talking about the 97 then 3.27 may not be the applicable gear ratio to start with...