There can be a little more to it than just bolting on a new turbo. That said, if your stock turbo isn't burning oil or making noise, and your engine isn't either, you can get a bit more boost out of the stock turbo. You should make sure you have a good "stage 0", or, that worn tune up type parts are replaced. Search this site for stage 0, and also check out peoples different builds. There are answers to all your questions on this site because most have been there, done that.
Or, citing your original post, if you want to know your boost levels while driving, boost gauges are $20 - $30 at the local parts store.