#1 Is it really relevant to what you want?
Plugins can do many things. Some plugins do more than one thing. Some are close to what you want done. So, first thing first. Understand clearly what you want to accomplish.
#2 Is it well supported?
Many people start by looking at a plugin’s reviews. I look at the support threads for the plugin. It tells me whether the author is engaged, active, accepting feedback, and fixing bugs.
#3 Do updates appear at regular intervals or is there long time between them?
There are good reasons why a plugin might not get updated frequently. If you see lots of issues and bugs in the support queue and infrequent updates, it gets back to whether the author is engaged.
#4 Are there enough people using it to cover “edge case” bugs?
Ideally, someone else finds a bug before you do. The more users, the wider the variety of environments the plugin lives in. Does that mean you won’t find a bug? No, but reduced the likelihood that you’ll be alone.
#5 What else have the authors done?
Does the author have other plugins? How are those supported?