I purchased a little gadget recommended by moderator jplante4, to check level of my tanks. It is about the size of your finger, and is pressed against the side of tank to check. Looking at Chris's pic above of tanks, when pressed just above level it lights red, and at level and lower it lights green. It actually works quite well, IMO. Sorry no picture, but a search may bring up thread that discussed it. IIRC it was around $25.
Jerry