Ask questions and learn from expert freelancers
1)Firstly see the primary information that is available about the employer, like whether he is payment verified ,email verified,phone verified. Payment verified is the best indicator that he/she is a legitimate employer.
2)Then check for reviews, if reviews are available , it is a positive sign, and based on the reviews u can choose to work or not work with him/her.
3) If none of the above are available it is a red flag, yet he can be a legitimate employer, but do not trust him/her blindly.
Do not trust him/her and do not start the work if,
he/she asks for a sample code on his/her own data or in certain cases your own data too,
he/she is reluctant in awarding u the project
he/she is reluctant in creating an incoming milestone.
he/she has a large number of open projects but still has no reviews or completed projects.
Nice answer by Shijudasan, Also ask about payment terms. Do not accept the project until the payment terms discussed and buyer deposited milestone. The moment you accept a project, freelancer will deduct their fee. such as for USD100 project you need to pay USD10. If the buyer didn't response and vanished then you've to pay at any cost US$10..Hope you understand.
Also take note charge the amount in milestone even it's a small fund. After delivery buyer might show you "I didn't like the job", "Not satisfied with the work", "I didn't like the design" etc. etc. The moment buyer say you'll loose the dispute eventhough you're 100% corrects. It happens with me. Trust me.