How to tell if there is poor water pressure before my main shut-off valve?
Work with the water company on this.  Unless you live in a rural area, they should be able to turn the water off to the property at the street. They'll then be able to pull the meter (something you can't do), and check the pressure there.  

If the pressure at the meter is good, it's a problem in the building. If the pressure at the meter is bad, it's a problem in their plumbing.  

Have you asked if they are doing; or have recently done,  any work in the area?  Or are fire hydrants in the area being flushed?