Guessing Boost Levels

[Alpine TSi]

Real quick question here, I was wondering if there was a way to determine what boost you were running without having to make a fourth gear pull on the freeway. What I would like to know is if let's say I go to the track and have my boost set at what I know to be 15 pounds, and want to run some race gas and 20 pounds. How I would I know I am at 20 pounds without cranking up the MBC and making pass after pass somewhere else just to see? Now I know I should have probably just set it to 20 before I got there and not laid into the way there, but that is not the point. I just want to know if you can make boost adjustments on the fly without buying some big old expensive electronic boost controller or dual stage(I would like to buy a dual but kind of like mine now) controller.

Thanks in advance.