If I were to ask you to contract your biceps, you could easily do it.
The pelvic floor muscles, on the other hand, are a little trickier to connect with. (It’s not just you!)
The Breath Cues Guide offers the idea of using bony landmarks to orient where the pelvic floor muscles are and what happens when you inhale or exhale. You may also be interested in this video demonstrating what happens with the pelvic floor during inhalation and exhalation.