It’s fortunate that I found such place where guys openly discuss “manly” issues.
Onto topic, I find it’s easier to just clench the whole crotch area for half a second each clench just for the feels of them. Once the muscles are a bit tired, you’ll notice the difference between the muscle surround ur anus (PC) and the muscle surround the inner base of the shaft (IC or whatever it’s called). Once they are tired and you keep clenching, they won’t be “synced” with each other anymore, one of them will contract first and the second one follows in a birsk moment. At this point, it’s much easier to isolate the desired muscle.