A few items I've bought turn out to be useful for combat, but most of them are just clothing and there appears to be no way to tell in advance. Darnitall.

Depending on where you're buying it, if you hover over the image and it has a tier number, then it can be used for battling. If you want to see how this looks, you can click on the link at the bottom of my signature. Everything there is used for battling so you can see how the tier numbers appear when you hover over them.

Thank you thank you thank you!!!

You're welcome. :)

Subetalodge is also a great resource for looking up items. It'll tell you if something is a Food, Clothing, Book, Minion etc. (it can sometimes be hard to tell which is which, as multiple items will fall under multiple categories).

Also, it'll let you know if an item can be earned through an Achievement, event, or otherwise (so you know not to buy it if you're aiming for it) and other useful info like that.