If it is a non SS monte carlo, the ratio will more than likely not be great. Here's how to tell if the codes are unreadable or missing:
Jack up the back of the car, put the car in neutral, spin one of the rear wheels, if the other wheel spins in the same direction, it is a posi. If it spins in the opposite direction, it is an open differential. If the car is a posi, spin one of the tires a full revolution, and have someone watch how many times the driveshaft turns, if it turns roughly two and half times, you probably have something like a 2.41 or a 2.56 ratio, the amount of times it turns is the rear end ratio. If the car has an open differential, you will need to chuck one of the wheels and keep it from turning while you turn the other wheel on the other side that is not chucked. Do the same thing and count the rotations of the driveshaft again.
I used that method when I bought a monte SS rear end.