If you have a routine every time you sit to meditate, you will have a systematic way of assessing your posture. After some practice the routine will become second nature.

1. Choose a cushion (or stools or whatever you use) which seems to be the right height, and sit down. At a certain height you will feel the correct balance of your body, which will make it easier to adjust your legs and back.
2. Check your whole posture, especially noting the alignment of your trunk from side to side and back to front. You can rock gently each way from your pelvis, if it helps, until you feel yourself to be balanced.
3. Now you are in a position to check more thoroughly for the basic sitting faults of slumping forward or arching backwards, and make any adjustments necessary.