So I am a complete newbie and was thinking about setting something up with a provider.  In my area we don't have a lot of girls that are reviewed here - but quite a few who advertise on backpage.  Any tips on how to make sure I don't walk into an LE setup?  I've noticed guys mention doing LE checks- how does that usually work?

You're really better off sticking with providers with reviews, it just makes it a whole lot safer!  That said, this ain't a perfect world and sometimes we don't take the safest route.  So then, avoid any discussion about sexual services.  Whether contact is thru phone calls or text messages, your only discussion (other than location, day and time) should be about the length of your stay and the donation for that amount of time.  Period.  It's illegal to exchange money for sex so you shouldn't be talking about exchanging money for sex.  As to the "LE" check often referred to, that's typically going to be someone getting naked or grabbing body parts, an undercover officer will not disrobe or allow you to grab her tits.

I have never done an "LE check" and have never been asked to, nor do I have any clue as to why it's a thing.

Join a verification service, check reviews, not just here, but anywhere. The LE check is worthless, if you show up, and there's LE there, you're busted. Just keep your mouth shut and call a lawyer.

The 'LE check' is worthless.  If the 'provider' is an undercover cop, you're screwed, and not in a nice way.  P411 will work with you to get you verified if you have no provider references.  While it's true that BP is often a crapshoot, there are some quality providers do use it with success.  Just do your research, stick to well-reviewed providers, 5-10 reviews at least, by reviewers that have numerous reviews on their list.  

etc. all in the name of investigation.

I'm also in an area that has few reviewed providers within even a two hour drive.  IMO/E, you have a much higher risk of getting ripped off than encountering LE.  You can minimize the risk a fair amount but it takes more work than just checking for reviews.
One of the key things is consistency.  If the lady has been running an ad regularly for at least a few weeks, she's likely not LE.  
Google the number and see what comes up, you may get nothing which means it's probably a burner#, google voice,etc, which by itself doesn't mean much. You may get a ton of hits you need to sift through that will give you some idea of what she does and lead to more options for research.  
Image search her pictures; if you get nothing, again it doesn't necessarily mean anything, but you may get hits that tell you she's been at it for a while and probably not LE, or she's got a bunch of fake pics.  The photo search may also lead to other numbers she's used which may then lead to reviews you can find.
BP is a crap shoot in many ways but you can load the dice in your favor if you put the time into it.  There are many safe, fun ladies that use BP, but there are plenty of shady ones too