A pressure gauge reading is one of the cheapest useful facts you can gather before talking to an installer. It takes a few minutes, needs only a gauge that threads onto a hose bib, and gives a plumber or technician something concrete to work from instead of a guess.
What you can measure yourself
Thread a standard pressure gauge onto an outdoor hose bib or a laundry tap that sits on the main supply, ahead of any treatment equipment. Open the tap fully with nothing else running in the house and read the static pressure. Then run a faucet or flush a toilet while watching the gauge, and note how far the reading drops. Write down both numbers, the time of day, and whether anyone else was using water.
Take more than one reading if you can. A single reading tells you little about a supply that changes during the day as neighbors draw from the same main.
What you should not try to do yourself
Reading the gauge is the easy part. Deciding what the reading means for your equipment is not. Whether a pressure regulator is needed, whether an existing one has failed, or whether the supply line needs repair are plumbing decisions. Hand those to a licensed plumber, and keep your readings in writing so they can see exactly what you saw.
Compare the reading to the equipment spec
Every treatment unit lists an operating pressure range in its manual or spec sheet. Pull that document before you shop. If your static reading falls outside the range for the unit you are considering, the equipment may not perform as advertised or may wear out early. Ask each provider in writing how they will handle a reading outside the range, whether that means a regulator, a different model, or a fix on the supply side. A provider who skips this question is telling you something about how they plan to work.
Questions to bring to the provider
Ask whether they will take the readings themselves or rely on yours. Ask what they will do if the pressure drops sharply during a draw. Ask whether the quote includes any regulator or pressure fix, or whether that is a separate line item. Ask what the warranty says about damage linked to pressure outside the rated range, because that clause often decides who pays when something fails.
Where the gauge stops
A gauge does not tell you about water quality, flow capacity, or what is building up inside the pipes. Those questions belong to a water test and a sizing check. A clean pressure reading is a good start, but it is only one input to the decision.