There are many reasons a pressure washer can lose pressure and a gauge can let you know there may be something wrong. Early diagnosis of a problem can save time and money. Using a gauge also makes it easy to determine the effect of different size nozzles on pressure.
Manufacturers of pressure washers used to put a gauge on almost every machine but now your hard-pressed to find a machine with a gauge on it. There are a couple of options to install one.
The easiest and best way, especially if you have a couple of machines, is a gauge you can click onto your pump with quick connectors. Using one of these allows you to check the pressure during your scheduled maintenance and determine if if you have a problem. A quick connect gauge allows you to check the pressure at your pump or at your trigger gun, showing the effect of various lengths of hose on pressure. A lot easier than making the calculation, as shown here!
You can also permanently fix a gauge to your pressure washer. The simplest setup is to install a "T" on the pressure outlet of your machine. Most gauges are 1/4" male thread so the branch of the "T" needs to be 1/4" female thread. The other 2 connections should be compatible with the connection threads on your machine.