the squeeze test. As I am inflating my float tube I have one hand on the bladder I am filling to make sure the bladder full but not over inflated. I also test when on the water by pushing with my thumb on the bladders to test the firmness of both bladders. If they are filling up in a hot day I can let air out of the bladder while in the water. (THIS IS NOT SAFE AND SHOULD NOT BE DONE WHEN ON THE WATER).
Finally the last method is something I just leaned and it makes sense, however I do see a problem with it. Check your seams, keep an eye on the seams of the float tube. Under pressure the will spread but they won’t break. So if it looks as if the seems are spread too far and are breaking then you have over inflated or your bladder is filling with too much air. The only issue I see here is most often the over inflation has already happened which means the damage has already been done. In all honesty all methods will help you to make sure you have the right pressure in your float tube, I prefer the squeeze test and you might prefer the PSI Gauge. It is up to you just remember we are all under pressure just watch it and have fun fishing.