Sunscreenr is a tiny waterproof UV camera (currently an already funded Indiegogo project, $93 plus shipping gets you one around December just in time for the dead of winter unless you live in the southern hemisphere) that records 30 seconds of video to show where you've applied sunscreen, and what spots you missed. Sunscreen coverage will appear dark, uncovered skin will appear light (this girl is not doing a very good job). Simple as that -- you'll never burn again! At least not until Santa brings me the flamethrower I've been asking for every year. So far the closest he's come is stuffing a Jesus candle in my stocking. The year I asked for a tank? Snow shoes. You've got some serious making up to do, fat man.
Keep going for a couple more shots (including a redhead who's about to fry) and a video.
Thanks to Stephanie B, who agrees the key to even sunscreen coverage is getting a hunk or bikini babe to do a thorough job.