Absolute easiest way is if you have a stock tank skid plate.
If the tank nearly touches the skidplate (as in so close you can't see daylight through it), it's a 31 gallon. If there is a ~2" gap between tank and skidplate, it's the 25 gallon one.
The 25's are quite uncommon though, particularly later, and on gas rigs. Why do you have to replace the sender?