Unless you have the password to his brokerage and bank accounts, I'm not sure how there is anyway to know this besides him being under oath and admitting it.
Either way, his finances are not your concern. There's no good way to bring up "you need to get your shit together before buying other people things". Even if you can't go through with it in good conscience, I would still do it as there really isn't a good way out. Broke are not, it's something he wants to do. It's like declining any gift: you're more offensive by declining.