No one can prove their role. Everything is conjecture because we never find out who did what until their role is revealed. Also, if someone claims to be [insert role here] that doesn't automatically mean that the real one is going to come forward because even if the liar is lynched, the real one is now revealed to the werewolves to be eaten.
Good reasoning for the Seer/Guardian/Alchemist/etc...
But it doesn't go up for the soulmates. As a soulmate, it's not that bad to get the werewolves focused on you. They'll attack one person every night, and it's either you or another innocent person. Heck, if it's not you, it could be someone with a special power. Also, if two people declare to be soulmates, and one is lynched, the other one is proven. In addition, two wolves would have to come out to act to be soulmates.
If you're a pair of soulmates, and you don't tell the world anything, you're wasting your role.
If you're a pair of soulmates, and you drop hints and pieces like Saffron and Trieste did, without actually claiming it, then you actually make yourself suspicious, while the real werewolves know exactly who you are.