You could check the generator/alternator for any nameplate, it should show voltage or a model number. Also see if there are any add-on gages that would indicate someone making modifications to the electrical.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy