I have some digital gauges that show, tems, temp range, hummidity and humidity range that came from Walmart or HD. Are they perfect, probably not but accurate enough to get a general feel for the room, its current conditions, and conditions over the past 24hrs. I have them placed at different spots in the room, different heights, etc and this has really helped me get a good range of temps and humidity. Like someone else mentioned if you have some condensation on the inside of your glass, you mist a few times a week and the tank is mostly closed your humidity range is almost definitely alright.
I think temp range is probably the most important to manage, temps in the 60's or 80's could be bad for your frogs, depending on species. Keeping temp ranges in the 70's has produced the best results for me, and that seems to be the general consensus on ideal temp range...