I read about this "hack" (hateful trendy word) on the internet a while ago, forgot about it completely and recently saw it elsewhere and had to try it because it intrigued me. It works. Plus, for some weird psychological reason which I am sure speaks words about me, I found it curiously satisfying.
I am not very good at gadgets or items in the kitchen that only have one use or that take up valuable space but don't get used all that often. For example, because I do not fry things very often, nor make candy and use the "finger prod" method to check meat doneness, I do not own a kitchen thermometer.
If you are frying (or shallow frying which is my personal preference when needs must), put one kernel of unpopped corn in the pan at the same time as you add the oil. When the kernel explodes into popcorn - there you have it! - oil perfectly heated to 350.