Drain some out and smell it and look at it under magnification.
OK... next SIP design I'll see if I can make a way to drain water out. For now, I have a clean eye dropper, and I pulled a little bit of water from the level indicator tube. That water has been pushed up from the res, so its definitely res water. The only magnifier I have is a cheap 30x loop I use to look for amber trichomes. I really didn't know what I was looking for, overall the water is crystal clear. But, I did see some little tiny ameba-looking-things swimming around in there. I'm not sure what to do with that information. H2O2 in the res? Or have I simply created a healthy living aquatic system? Since the water is in contact with soil, aren't some micro-life-forms inevitable? I tried to smell it from the fill tubes, got no scent at all.
SomeGuy, you ever look at your water under magnification?