Extremely low budgets are one of several warning flags. Some of my clients came via Upwork, where I was very careful – I denied numerous invitations to submit bids that were unreasonably cheap and opted not to submit offers for those in the open feed.
There are several. Before I work with a client, I prefer to get to know them. If I can’t, it’s money up front or nothing happens. Look out for psychopaths and sociopaths as well; there are a surprising number of them in business.