How do I know if I am putting to much pressure on the horse reins when I ride?

This depends on your philosophy and the style of equitation you are pursuing.? German style dressage is heavy – the pressure easily measured in pounds at lower levels, given certain assumptions in the training progression.
Traditional American-style “Western” equitation (not Vaquero style) often uses no pressure – but forfeits consistent contact (and in so doing, any notion of self carriage and actual collection).? Baucherist-style dressage (which is my denomination) maintains contact with a pressure measured in ounces, or, in effect, merely the weight of the reins (similar to the Vaquero product).? Which is “correct” is, of course, a never ending debate.
If your horse is “resisting” (pulling on the reins, shaking is head, over-reacting), then either the pressure you’re using OR the way in which you are applying it (the fidelity or tact) is unfamiliar to the horse – either because he is yet untrained, or is trained to a different style of equitation than the one you are using.? So your issue may be “pressure” per se, but it could also be “approach”, or merely an “education gap” on your part and/or the horse’s.