The usual cue is the feeling of stopping wind, or of interrupting urine mid-flow. Use either only to identify the muscles, and do not practice on the toilet. The muscles sit between the anus and the scrotum, and a correct squeeze feels like a gentle lift rather than a downward push.
If you are unsure, try lying down with your knees bent, where a small movement is easier to notice. Many guidance pages also recommend keeping the thighs, buttocks, and abdomen relaxed, so a hand on your stomach is a useful check. If the hand rises, you are bracing with the belly instead of isolating the pelvic floor.
A helpful test is to hold the squeeze for two seconds, release fully, and repeat three times in a row. If the third repetition feels the same as the first, you have found the muscle and you are not overworking it. If the third feels vague or shaky, the effort is too high.