There are many places that are good to check level...as LJAZ mentioned, the easiest is just inside the door. Another is a counter top, but, the whole reason for "leveling", aside from comfort, is the refrigerator, so, checking inside the refrigerator is "best". That being said, "level" is a misnomer for most purposes. "Comfortable" is good enough. If you are comfortable walking, sitting and relaxing in your RV, then it is level enough. Refrigerators, despite a lot of "scare" information out there are quite good to go at any angle that you are comfortable living in.
The best thing to do is to mount an external leveling device...the kind LJAZ uses or this one (shown mounted on my old 210WBS, sniff...):
It is large enough to see in my rear view mirror and that makes it easy to jockey the trailer around a bit to find the spot that is most level, and failing that, it indicates by the markings how much you need to raise which side to make it level left to right. It also has a little level that sticks out the front of it that you can use, with the tongue jack, to level it front to back. Take your RV to a parking lot, or in your driveway for that matter, and using a level inside the RV, as above, make your trailer level. Then, once the trailer is level, attach the external level so that it "reads" level. Then wherever you go, it will tell you whether you are level and by how much you may be off.
Here's that level at Amazon:
Camco RV Level
I use that same level on my 5th wheel and have found them to be very reliable and sturdy.
Now for level...there is no need to be "dead level" and several reasons why you might not want to be dead level. Your refrigerator, as mentioned is quite comfortable running at any level that you are comfortable living in. If you are comfortable cooking, walking, sleeping and relaxing inside your trailer, then you are actually level enough. There are a couple of reasons that I never set the trailer up level for. The first is if you are camping in weather that you need your A/C on, then you have to deal with cold condensate running off your roof. I don't like dripping on head or neck. Another reason is rain. If your trailer is dead level and it rains, the rain water won't have enough "running force" in most circumstances to rinse dirt off your roof or move leaves off. Also if dead level, you will probably find that water will puddle in some areas of the roof and slide(s). To fix both those problems, when I set up, I always make the curb side (where your door and awning are) ~1/2" higher than the street side and I make the tongue ~1/2" higher than the back end. Inside, you can't feel the 1/2" off of level and your refrigerator will run just fine, but, outside, the A/C condensate will run of the street side rear corner where very little happens when you are at a campsite and if it rains, that 1/2" slope will make the rain water also run off of that corner and fast enough that it will "rinse" your roof in a good shower and won't accumulate many, if any, puddles, particularly on your slide.