A few ideas.
1st, You have to know why people buy POD products that you want to sell, in your specific niche(s). If its a t-shirt/hoodie, do they want to support something? Tell the world something? Is it their fanatism, is it ego, is it status?
2nd. Find successful POD products in that niche and study it, look beyond the graphic, dig deep, why would someone buy it and wear it? read the reviews.
3rd. After clearly identifying the niche, their desires / motivations either: Create a similar product but different design or reapply the concept to another niche (for example Cats instead of Dogs)
Also I'd advise you to look at best selling POD products of the kind you wish to sell, and try to identify any shared visual elements, font, colors, structure, punctuation, word length, female or male. That way you can increase your chances of success.
There's multiple ways you can validate the products, either drop some money on ads and run a few carefully researched and designed products, or make a TikTok account and utilize something more of a "shotgun" approach and post a ton of different designs, then get the most disproportionately successful ones and further test them/try sell them.
From my experience and knowledge, POD can be a super tough niche to become successful in, due to such low barrier to entry, anyone can use T-Spring and such.