It's tricky to know if you're doing them for sure. When I had really poor muscles I'd have to hold my breath and really concentrate to do them, but it does become easier the more you do them. Usually it's said if you stop your urine flow then that's using those muscles, but don't do that often, it's not good for you, but just try a couple of times to get a feel for those muscles.
When you orgasm and your muscles clench and unclench that is the same feeling of those muscles being used so sometimes that's an easier way to know if you're doing them correctly because you might be more used to that sensation than of stopping your urine midflow hehe.
Try to relax when doing them, not clench your bum, leg or stomach muscles but really concentrate on those deep deep muscles inside and you'll feel them almost lifting. It's not the easiest to start but give it a week and you'll notice a bit of a difference and it'll become easier and natural and part of everyday life. It's really good to work them even when not pregnant and continuing to after will be good for when you're older too.
Hope that helps x