Most Helpful Girl
Easy. Date your female coworkers or date women who make the same -or more- amount of money that you do.
Most Helpful Guy
Just accept and deal with the fact that there is no such thing as an "absolute" guarantee in life.
How does a girl know if you're not just with her because of sex? She doesn't. You can get married, get bored of her (sexually), and then get divorced. So, at the end of the day, what "guarantee" does she really have?
And if she tries to "test" to see if you're not just with her for the sex, you know how that goes. You (as the guy) start thinking, "I guess she's not interested in ME, or in SEX, or HAS ISSUE WITH MEN, or is PLAYING GAMES, and I don't want to take on the risk of being in a SEXLESS or SEXUALLY MISERABLE marriage/relationship," and you move on. Did you move on because you didn't have sex? No. So, the "test" failed in its design and results.
I simply use that as an analogy so you can see it from the analogous flip side.
As a guy, you simply have no idea whether a girl is simply with you because of your money. People can fake entire relationships. You simply don't "know" with (100% certainty), ever. Just accept that, or else you'll drive yourself crazy.
You can "not get married," sign a "pre-nupt" or "cohabitation agreement," etc., but you still never "know." You can "not spend a lot of money" on her, and "test" to see whether she offers to pay for stuff, etc. But, this are equally and similarly poorly designed tests (i. e., crappy tests).
It's more likely to make the girl feel like you're cheap, or that you're not that interested in her, or that you have issues with relationships, or that you have trust issues with women, etc. than it is to provide you with reasonable assurance that she's not just with you because of your money.
If it's one thing I've learned about "con-artists," it's that they're more sophisticated than the child that steals the cookie from the cookie jar. You can invest your whole life "protecting" and "guarding" yourself, and you'll still fall victim to such a person once or twice throughout your lifetime.
And at what cost? Nothing is "free" in life. Each choice has its consequence. If you put up walls, then you are also "excluding" a whole bunch of women who are not just interested in you for your money.
Additionally, appreciate the difference between a woman who is "JUST interested in your for your money ONLY," versus a girl that "also appreciates you or your money." There is nothing wrong with a woman wanting a man who has money - that's perfectly normal, and you should feel good to be such a guy.