Since this subject keeps cropping up again and again, I decided to compile a few tips.

I already gave some general advice on how to find a good language exchange partner here:

https://www.italki.com/discussion/117648

To this, I would add these tips:

-Go to the other person's profile and pay close attention to his/her community activity: See if he's really been helping other people, writing posts in your language for correction, and/or taking paid classes with teachers. (But as Grzybek said, some people don't do any of these and still turn out to be great partners. Thanks, Grzybek!) 

-Carefully check if he's often one of the first people to reply whenever a girl posts "Hi! Klingon girl looking for an exchange partner here" even though he's not learning Klingon! That should be a red flag for you, because it means he's not here for learning languages.

-Look up his profile picture using a reverse image search engine, such as Tineye and Google images, to see if it's not stolen.

-You should be aware that other people can do the same with YOUR picture, so maybe you can change your picture to a cartoon or something, as is often the case in other forums. Looks are not important here. (But as Tety said, other people might also want to see the real you instead of a cartoon. Thanks, Tety!)

-Similarly, there's no need to use your real name here. A nickname is OK. Make sure it's not a nickname you use in other accounts. It can even be random characters. Have your cat choose a nickname for you.

-Be brutally honest and specific about what you're looking for and what you're not looking for. Put it on your profile. State clearly that you won't accept exchange requests unless the potential partner meets X, Y and Z, and that the potential partner must propose a specific date and time for exchange in the first message because you're too busy to deal with "Hello, how are U?" back-and-forth nonsense. If you receive a message from someone and it's pretty obvious they haven't bothered to read what you wrote on your profile info, why should you bother to read their message? That's what the delete and block buttons are for.

-Create a disposable Gmail account with a nickname (not your real name) for exchanges. Never give your real email address and, please, please,

-NEVER give your phone number or WhatsApp number. WhatsApp is a suboptimal language exchange platform (Thanks, K P!): You rely on poor internet connections (most people use it without WiFi), you work with a tiny screen, you can't chat and have a video call at the same time, you can't share your desktop to show vocabulary lists, you can't help with essay correction, the conversation can drag on for ages instead of being limited to a specific time, etc. And if you have to change your number because you keep getting messages from a stalker, it will affect your life, maybe even making you lose work opportunities.

-Instead of WhatsApp, Skype and the like, use Zoom (www.Zoom.us) for exchanges. Sign up using the disposable Gmail account you have already created. With Zoom, you can 1) Easily record the call and have the recording as proof in case of harassment. You MUST warn the other person beforehand, and remember that Zoom shows a recording warning in red during the call. (Thanks, Berze!).  2) You can share a window with a PDF without having to show your entire desktop. 3) It's more stable than Skype, even in China. 4) You can have a call between 3 people: You, your partner and a friend of yours.

-On italki > Account settings > Privacy > Contact permissions > "Who can send me a message", you can change the setting to "Friends only". You might also want to consider unmarking the two boxes in the "Friend requests" section if you prefer to always initiate contacts yourself and not having random people contacting you.

-Don't give up!