1) Buy a portable blacklight. You don't need to get the full-on purple wand they use on Inside Edition. Pocket-sized ones are available online for less than $20, and will allow you to fully inspect your hotel sheets before jumping in. That said, if you find they're dirty, then demand a room change, and those are dirty, then change hotels and find THOSE are dirty too, we can't be responsible for your overnight stay in your backseat.
2) Don't stay in a hotel. Simple enough, right? It's 2016 and Airbnb is what all the cool kids are doing anyway. Then again, there's no guarantee your host is washing his or her sheets either. But since they don't have 400 rooms to clean, your odds might be better.
3) Bring your own sheets along. Does that immediately make you known amongst your friends as "that guy who, no joke, brings his own sheets to hotels because of something he saw on Inside Edition"? Yes, yes it does. But it's not completely unheard of, and might be the only way you can guarantee a clean bed.