Continuity
Use a continuity test at first. If you have continuity, you'll know voltage will pass. That said, you can have continuity and still not be able to pass the required voltage, so this is just a test. Once you get all your wires in, you'll be able to load test with the actual battery.
If you want, you can wire in a tiny 9v battery and use that to test your bulbs and such. They won't be bright, but you'll see a glow at least. Obviously, it won't be able to power the wiper motor or headlamps, but the other low amp draw circuits will work. And the amp output of that 9v is so low, that you won't start any fires if you get something really wrong. But in reality, you don't really need the 9v setup since checking continuity will get you on the path to start testing with the real battery.