I assume you mean "thermometer" not "thermostat" most folks use one of these. Pipe Thermometer They are cheap and whilst not accurate to a degree or two are adequate for balancing requirements. You can also keep it for future use. A more accurate digital instrument would probably not really offer any any advantage as there will likely be a variance of several degrees in the flow water temperature as the boiler cycles on and off (unless you have the latest modulating type) anyway.
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