Jack up the back of the car and put it on stands. Mark the pinion and rear end with a line and then spin the right rear wheel one revolution while counting the number of times the pinion goes around. The trans will have to be in neutral. If the pinion goes around 3 1/2 times you have a 3:43 - 3:55 approximately. A little over 3 1/2 and it could be a 3:73. Under 3 times and it would be a 2:73 or so. This will get you in the ball park but is not as accurate as pulling the cover.
Mark