“If he sees that Hashem is providing his needs then he can assume that he is L’shem Shamiyim. I’ve meet some many Bnei Yeshivas that can’t explain how they get by, just that every time they need to pay a bill or purchase something, etc. the money just comes (and not from the government).
If Hashem is not behind them than why is he constantly doing miracles for them?”
This phenomenon is not limited to benei yeshivos.