How do you know whether something is a waste of money?

Some purchases seem exciting at first but then turn out to be dull and rarely used. Those purchases’ fate is usually fire sale (at garage sale, ebay or otherwise), donation, or the trash heap.

Some purchases seem dull at first but then turn out to be quite useful and practical.

So, I’m wondering, how do you know whether something is a waste of money? Is there a thought process or checklist you use when deciding whether to make a purchase?

Note: I am mostly referring to everyday items like clothes, gadgets, toys, electronics, etc. Though, I suppose this question could extend to bigger purchases like cars and houses as well.