[Evans, Marv]

Buy a water pressure gauge (they're cheap) and check your incoming pressure like Dale says. Next check if the shower heads are clogged with mineral deposits. Then check to see if you have a drop ih pressure at the shower heads (you can do these at the same time). You can check for a slow leak (in the plumbing) at some point by checking the pressure at faucets going away from where your water supply comes into the house. You will figure it out once you get started.