uh...simply give them the money they need if they don't declare having enough ressources maybe?
I mean I'm not sure what you expect, at one point you have to trust the citizens, on 1. their net worth they declare for taxation 2. consequently their needs to get help or not
if someone declares to have an income of 0, mention he has life issues, sign up for help, then he needs money to eat. you can have it on a form online.
still would cost far less than ubi
and if you make a law with a huge fine in case of fraud, but don't actually control it, you will waste no ressource on bureaucracy. you will only fine people on rare cases where the state discover a fraud by chance. this is more efficient than ubi.
almost like we do in my country. it works. except the state is giving way too much, they give more than the necessary to anyone. but that's another story.