It's hard to know for sure, but here are some steps off the top of my head that can help you figure that out.
-Google their name/company
-look at their reviews/interactions on this site. obviously this isn't always useful, because lots of trustworthy people make an account just for a one-off job, and some people use bots to artificially inflate their positive reviews.
-look at the language they use. bots will usually use long run-on sentences that ask you to visit other sites, are usually free of spelling errors, and sound robotic. but again, this is also not a perfect method because many people on here don't have english as a first language, and talk in a stilted manner. so again, use your judgment.
- besides these tips, there really isn't any fool-proof method I can think of to prove that a client is or isn't real. use your judgement, and if a job makes you uncomfortable, then don't do it. there are like 100 jobs posted every minute on here. plenty of trusty people for you to choose from.
Hope that helped a bit, good luck and stay safe out there! ;)