If your butt is too padded (as mine seemed to be...) then sit on the floor or other hard surface and rock back/forward a little and you should get a sense of where the main pressure from your bones is.

Alternately you can just grab your butt (or get the person who likes to grab your butt to fondle it for you ) and rub/massage gently and you should be able to feel the point of your bones from that...

Remember that your sit bones are part of your pelvis, and as such will be somewhere above the top of your thigh bones... what I'm trying to say is don't look for your sitbones on the widest part of your butt cheeks...

Have fun!!!