The best method is to monitor the pressure & temperature drops.
Most new heat exchangers have been designed to have a maximum pressure drop of 5 psi. (Delta P). The maximum temperature drop systems are designed for is 20º F (Delta T). This maximum might occur on a heavy demand i.e. (a sports team using showers at the same time after a game).
When there is low demand i.e. (during the game) the temperature drop may only be a few degrees to make up for heat losses that occur as the water is recirculated thru the piping loop. The closest the average temperature approach equals is 3ºF.
If the heating source temperature is 160º F, the closest the second liquid being heated would be = to 157ºF.
There is also a 1:1 ratio between the two liquids. If the inlet of the heating liquid enters @ 160ºF and the second liquid enters @ 154ºF and exist @ 157ºF the heating exiting temperature = 157ºF.