The saddle is the symmetrical interface between an asymmetrical horse and an asymmetrical rider. Remember the purpose is to spread weight evenly over the back so if the saddle is not straight the weight will be uneven, the horse will have to compensate and the asymmetry will worsen. The horse can only develop muscle evenly if the saddle does not restrict.
To check the general symmetry of your saddle:
Stand with your feet together. Place saddle on your feet with the cantle facing upwards. Check the centre of the cantle is in line with the centre of the pommel.
In the same stance as above imagine a centre line down the gullet and check that the panels are symmetrically spaced either side of that line.
Turn the saddle round so that it's pommel up and do the same check. It maybe straight at front but not at the back or vice versa.
Place your fingers in the stirrup bars and lift the saddle.
Is it balanced?
Does the saddle tip forward or back or to one side?
Is one stirrup bar further forward than the other?
Are the stirrup bars tight to the panel?
If you find any issues with your saddle or if you're not sure please consult your saddlefitter/saddler.