I've recently received a Steam friend request from someone I think I know, but I am not absolutely sure. Sadly, he has no useful information in his profile page. Is there any way I can get in touch with said user to try and figure out who they are before deciding what to do with their friend request?
A reliable way to contact another user who is not a friend is to get into a small group with them. If he's not part of any small groups that you can join, you can create a group and invite them to the group.
Then you can navigate to the groups under your friends list and start a chat like you would with a friend.
You can send them a message via the Steam Protocol:
You will first need to find out that users 64bit ID, which you can do via http://steamrep.com. With this site you can also see if they are trade scammers or not.
Then add the 64bit ID to this URL:
Then execute it via Win+R or in your browser.
This way you open a chat with them. However, they will get a notification that asks them if they want to answer your request. If they added you anyway, they probably take it.
Steamrep.com even provides a link for that feature in their search.
They have to be online for that though.