They don't do free trials and you can't "test out" the search without paying. The dev said it's because they're really small and don't want to collect more data than necessary to fight off abuse. So you do have to trust and pay that 5$ to try them out.
But once you do it's a cakewalk to verify that theyre not just a bing frontend or whatever it is that you're worried.
At the end of the day they're a company and you can't verify anything 100%. If you're this paranoid then maybe it's not for you.