Low Water Pressure– not a Main problem?

The city of Chicago has determined that our low water pressure is not caused by the water Main. Our symptoms are: low water pressure in all faucets, hot and cold, gets much worse if a faucet or toilet is used while showering. Our house is not very old– 15 years, so pipes should not be an issue, I am told. What are the logical things to check before I call a plumber or city plumbing inspector? Also, we are not certain the City has made an accurate determination regarding the Main, but do not know how to verify this. Can a plumber do this? Thank you.