you can't test not tolerating the sun without literally dying in the process
Yes you can. Just stand in the dark and stick your little pinky finger out. If it starts to sizzle that's pretty compelling evidence. Pull it back and if it starts to heal at a much faster than human rate, then that's double proof.
Put your scientist boots on and there are a whole series of experiments you can do.
- Order Italian take out with EXTRA Garlic
- See if you can hold your breath for over 5 mins
- Can you bench press/lift far heavier weights or even do more push-ups than you could ever do before
- See yourself in the mirror
- Knock on the neighbors and see if you can enter without invitation
- Try touching a crucifix
- Dip your finger into holy water
- Catch a fly with chopsticks - if you succeed you're either Jackie Chan or a vampire
etc.
Now some of that would prove nothing as the myth/lore on vampires one knows could be errenoros, but hitting the jackpot on one or two is more than enough evidence that one is no longer human.