How do I verify if a freelancing offer is legitimate?

Hey all! I have a person who pinged me on LinkedIn and says he has a freelance opportunity for about 800 to 900 usd a month. I asked him how much time I need to spend per week etc and he gave clear answers. Seems fair and I am ok with working more in the beginning of that gives me a second source of income.

The only thing is, he only pays monthly and if he is a scammer, I would have worked for a month for free. Anyway to verify his offer's legitimacy? How do you guys handle such things if you have freelanced before?