The easiest way to check this if you are on your own, is to take your feet out of the stirrups and bring both legs up so that your knees are touching in front of the withers, as if you are a jockey riding super-short. Then you will be able to feel your seat bones very clearly: whether they are exactly the same distance from the middle of the seat of the saddle, and whether your weight is totally evenly distributed over them. Wiggle around a bit to be sure, then stretch both legs out and let them flop down as long and relaxed as possible.
Not to be done on a very lively horse though, as it makes you very precarious for a few moments!
