Like several others mentioned, I use a small level and place it either on the floor or the counter. Typically do that first thing as soon as get into a campsite, before extending jacks, just to see how far off level it is before I begin. Most times I end up using blocks under each jack, and sometimes drive one or two axles up onto blocks as well to get it level.
When it comes to leveling, don't be lazy and just let your jacks do all the work. Carry with you, and use blocks of some kind. The less you have to extend out your jacks when leveling, the more solid and less 'shaky' it will feel inside (not to mention its easier on your jacks). I typically get as close to level as I can with blocks, then put whatever remaining blocks I have left under the 4 jacks so jacks don't have to extend as much. Only then, do I extend the jacks to finish up the leveling.
I never use the 'Auto' button, either, unless we're on a very level site, like a paved one. I've found using the 'Auto' button will make it go through several unnecessary iterations of up and down, and end up raising one side up much higher than necessary. It does not work well unless you're almost perfectly level already. Manual mode is much better, IMO.
This reminds me of last time we were camping, about a month ago. Pulled up to our campsite (was a back-in site), older couple were sitting outside across the road from us, heard him say to his wife, "Oh, good, our afternoon entertainment, watching this young couple try to wrestle that big MH into that site! They look young, inexperienced, this oughta be fun to watch". Unfortunately for him, it ain't exactly our first rodeo, we been doing this for many years, haha. :)
Anyway, got in the site pretty easily and quickly, nothing really to see. Site was particularly unlevel, so I had to use a lot of my blocks, including some under the wheels. Afterward, got to talking to them (neighbors that had made that comment), his wife said, "You just gave my husband a good education! Like how you used blocks to do most the leveling, most MH owners just take the lazy way out and make jacks do all the work. Also like how you made those blocks with straps on them to make them easier to carry! I want to see how you made them, I think we're gonna make some of those." (I was using a set of 'homemade' blocks I made from pieces of 2x10 with straps on them to make them easier to carry).