Would it be rude to ask point blank if they have the money? Someone asked us if ‘it’s’ a good liveaboard - and did we have a recent survey, because their marina requires it. okay...
I would not regard that request as rude at all. "Will you be planning to pay cash, or are you pre-approved for a loan?" seems polite enough.
If they waffle, it's a good warning sign. People who can easily pay will be happy to tell you as much. Others may just lie, so then...
The more aggressive form would be to as for "proof of funds to close" (like a bank statement) - which happens in real estate and business and would be a bit much before you had shown the boat to somebody. But if you're about to countersign a contract, that would be a fine time to ask for documentation of funds-to-purchase, IMHO, to ensure your time is not being wasted.
$.02,