I noticed the picture on the boiler literature does not have a pressure/temp gauge installed on the system. How does an installer determine pressure in the hydronic system?

A pressure/temp (P/T) gauge can certainly be installed in the piping. Manufacturers often includeP/T gauges with their manifolds as well. The NextGen Boiler is unique in that it has a pressure sensor integrated in the boiler. The pressure reading in PSI can be viewed on the control panel. This will eliminate the need to purchase and install a pressure temp gauge thus saving on labor. Note that in order to read the pressure in the system, the boiler will first need to be powered with 240 VAC power.