When you are charging or just checking a refrigeration unit, you use a set of gauges. The blue hose is connected to the port at the low-side of the refrigeration system and the red hose will connect to the high side of the system.
To properly know what your pressures and temperatures should be, you will need to know what refrigerant you are working with and a “Pressure\Temperature Chart” (P/T Chart).
With a P/T chart, if you know a temperature or a pressure of the ambient air or the refrigerant in your system, you can use a P/T chart to convert it to the equal pressure or temperature.
For an example using the chart at the right, at 100°f R22 refrigerant pressure would be 198.4. R502 at 100° would be 218.6, R12 at 100° would be 119.4 lbs pressure. If you just know a pressure, cross the pressure on the chart to the corresponding temperature.