Well the best way is to spray them occasionally. They do sell humidity gauges. The one I have is a thermometer and humidity gauge in one, so I don't have to worry about a separate thermometer. It's by Zoo Med.
You have to get things that keep humidity in. The soil I have in my tank holds humidity well, and the climbing wall is also designed to hold humidity. Without those things, you just need to spray them more often.
Those heating mats work fine. If you don't have heated rooms, it's necessary since their temp range is basically the same as Bettas. Mine stays stable in the right range because it's right against the heating vent.
For lids, mine just has the standard terrarium mesh lid with a basic 10g led aquarium hood over it to keep humidity and heat in. Any good you get needs to be able to keep it hot and humid, but also allow fresh air to get in.
PetSmart sells a kit that has the 10g and all the basics for $56. The only thing the kit didn't really have was climbing toys, which I has to buy separately. It came with the two water conditioners, food and water bowls, the mesh screen, food, mineral blocks, soil, and sand. The only thing you may need in addition to that is another bowl for the salt water since you need one for fresh and one for salt. This is the kit.
Zoo Med? Reptihabitat Hermit Crab Habitat | reptile Terrariums | PetSmart
I know it says $61 but at my store it was $56 on the shelf. It may differ by state.