Hi, is there a way to check the bike display without a battery? I bought the bike missing the battery and don't want to buy a new battery if something else is wrong. Someone told me to hook a DC power supply to it to test with same voltage. Bike is a Rad Rover 6 Plus.
That would work; you need a 48V power supply.
you should probably check the serial number on some of the stolen bike sites like Bike Index or Project 529 - if a bike is sold without a battery, it's not unreasonable to suspect the bike might have been stolen.