How to tell if water pipes are grounded?

My cold water piping is bonded to the panel at the cold supply of the clothes washer. The hot piping is presumably electrically connected to the cold through the brass mixing valves of the shower and tub. There is negligible resistance between the cold and hot pipes at the washer supply valves.

Following Tyson's link I see my kitchen and lavatory faucets are not bonded because they are connected with non-conducting flexible supply lines; an ohmmeter test confirms this. I tested this by plugging an extension cord into an outlet and measuring the resistance between the ground of the cord and the point under test.

As long as there is water in your plumbing system, there is no way to electrically determine if your electrical system is bonded to your plumbing, since the water inside the pipes (even if PEX) will conduct electricity such that any two points tested between electrical ground and plumbing will be close enough to zero to count as such. To perform a test with an ohmmeter, you would have to drain the plumbing system (generally not really that difficult) and then do the test. I would do the test between the load center ground (case is good enough) and incoming water supply pipe. Doing the test between the load center and some other point in the plumbing system would not reveal if the bonding was done at a point in the plumbing on the house side of a flexible sleeve, which, unfortunately, is done far too often.