The best times to check your glucose levels are before meals, 2 hour after meals and before bed.
also, it is necessary to check the blood sugar level the first thing when one wakes up in the morning, before doing anything else,
but, these are general rules which might not apply to each individual and the best is to follow the recommendations of a qualified medical diabetes professional for your particular state of current diabetes condition as each persons needs vary, that is the frequency of your measurements,
and as for the exercise, before the exercise and it should not ideally be below 100 mg/dl and if it is above 200 mg/dl, no exercise is to be done.
it best to test the effects on blood sugars both before and after the exercise to see it's effect on your body.