Because the chances are you can point to the thread where you signed up for the sig campaign that will likely have your wallet address. If it's a third party site that handles the sign up process then screen shot it in case it disappears. Then you can point to your wallet address with every single payment arriving preserved for all eternity in the blockchain.

Many campaigns also have vanity addresses that identify their origin. Keep all your old private keys. Makes notes of what you signed up for.

There is literally no better proof possible. It couldn't be simpler. Just because you've wound up with a large amount of money in no way makes it 'wrong'.

If I'd spent $500 on 100,000 BTC back in the day and had a bank statement and wallet movements to prove it there is no nefariousness involved despite the ability to buy the entire bank.