While upright, and at slow speed at which you know you cant fall or loose control, and on various surface you expect to ride, try to find the rear wheel blocking threshold. (and very carefully the front wheel blocking threshold if you dare or need more feel from the hand). Assuming you know your bike on dry surfaces and you remain objective, you should be able to have a feel of the traction available. backoff a bit from this braking intensity further cause at higher speed, the water doesn't clear as easily.
Also try to start on painted lines a few times when alone on a stop (upright again of course ), so you can understand how easily it looses traction.
Also try to compare tire tracks with center of the lane, tire tracks are cleaner, but the dip contains more water.. Center of the lane not as clean, but possibly not as flooded, especially in city streets with lots of big trucks carving the pavement.
Feeling he force of breaking and starting at threshold is a better way imho than testing the limits in a curve at whichever lean angle...Every ride, rainy or not, there is always 2 seconds here or there to do such test. Build your comparison base.
PS: the foot test is BS, sorry. you have to be quite experienced, very used to do it, with the same clean sole boots (no mud, no grease, no grass, etc. to feel anythng that you might interpret very approximately anyway. About as good as touching the ground with your gloves....