not hitting full boost

[HiImBrian]

Well I just got tuned a few weeks ago and we set my boost at 21 psi. I just noticed yesterday when I was driving home from school that my car is only boosting to about 17psi. I was thinking either boost leak, or something with my mbc or external wg. I've checked all the lines for the mbc and wg and they are all in tack and look fine. I don't think there would be a problem with my wg because it has been working fine for a while now and it doesn't look to have any problems. So I am thinking by process of elimination that I am dealing with a boost leak somewhere. Does this sound like a correct assumption? I would like to know before I went and got a boost leak tester. Thanks