How to know if I can change the prefix & suffix in an item before I buy it?

[David Goggins]

I'm new to the game and looking to buy expensive gloves but I want to be able to atleast add minion damage to them.

[Grumpy]

crafted stuff have a blue text font like here https://www.pathofexile.com/trade/search/Betrayal/0aeQhg

P and S at the end of the line tells you if it prefix or suffix
you can find out here https://poedb.tw/us/mod.php if something is prefix or suffix

alternativly, you can also look for open affix in the search options within the mod filters category