The charging receptical way can work in somecases. But sometimes the two prong receptical can be used on 48 v. For example, I converted my '92 up to 48v, and wired it so it can charge via two prong plug. I just purchased a two prong 48 v charger from Lestronic. Here is the tried and true way to tell... Open up the vent caps from the batteries like you are going to fill them. If they have 3 holes each, that is a 36v bank. If it is 4 holes each, that is a 48v bank.
How to tell if it is wired for 36 or 48? There are a few ways... First, look at the motor, if the sticker is still readable. It should tell you the specs of the motor, which should say whether it takes 36 or 48 volts. Occasionaly, you will find a motor that takes 36 to 48 volts (which are great if you upgrade from 36v, because you don't have to change the motor) but anyhow, if it was the 36~48v motor, that won't tell you much. Here's another way... Does it have accessories? If so, trace the wires from the accessories and see if they are made to go right into the battery bank, or if they go through a voltage reducer box. If they go right into the batteries, it's wired for 36v. If it is wired to go through a voltage reducer box, it's wired 48v.
Let me know if this helps. If not, check the solenoid. There will be a little diode on the small wires, and a larger resistor on the larger wires. If any numbers on the diode and resistor are still readable, let me know what they are, I can ID the wireing that way too.
Hope this helps!