I bought my rain barrels at hardware stores (mine look like this). I know you can make your own, but this was easy and I didn't have quick access to a barrel no one was using. The weird thing is that neither of mine have removable tops. And since they're opaque, I was always wondering just how full they were. I asked some friends with similar rain barrels how they checked the level and the normal response was "why do you care? They either have water, or they don't have water."
Well, I was curious.
So I did some googling, and the responses were a mix of "why do you care?" and then there were complicate solutions about clear tubing on the outside, float systems, additional holes, plumbing... it got weird.
Which is why I wanted to put up a quick post about the obvious solution I found, which will seem way too obvious, but if it was a total "duh" I would have found it googling, right?!
Commercial rain barrels usually have a spout at the very bottom for drainage that you can hook a hose up to.
If you hook one end of a hose to that spout, and set the other end of the hose on the ground below the rain barrel, water should come out the hose.
If you lift the free end of the hose above the level of the top of the rain barrel, water will not come out. Gravity and everything.
Somewhere in between those two points, there's a transition... where if you lift the end of the hose above that level, water isn't coming out, below that level, water is coming out.
That's the water level in your rain barrel.