I've been going down the learning road with two broken printers that I'm rebuilding with better parts and electronics.
One thing that I've recognized is that there is a pretty low likelihood that any hotend or heatbed that has had the thermistor/thermocouple and/or the printer board swapped with a non-OEM part can be trusted to accurately report it's own temperature.
Sure, there are lots of things I can (and do) do to try to make it as accurate as reasonable like calibrating with thermistors from multimeters, IR thermometers, etc., but each method has limitations. You never know if the 2nd thermistor is mounted both correctly, or if it is reading the same local temp as the printer thermistor. IR thermometers have issues with reflective surfaces (like aluminum hot ends and build plates) Calibrating the thermistor constants from experimental data isn't perfect.
IMHO, any hotend/heatbed temp on a DIY setup might be off by a constant +/- 5C or so, more if it is poorly calibrated.
Printers use PID controlled heaters to keep oscillations down to a degree or two Celcius, because people say it impacts print quality.
Is there a good visual or experimental way to know whether your temperatures are "correct" for your printer/filament? IOW, if my filament was supposed to be heated to 220C, how would I know if my printer was having issues because the "true" temperature is only 215C (or 225C) when it is reporting 220C?
One common problem I've experienced is the nozzle clogging after the transistion from layer 1 to layer 2. (Layer 1 = higher heat and slower speeds, Layers 2+ = lower heat and faster speeds.) It's been a struggle to know which factor (lower heat or faster speeds) are to blame for the clogs after the transistion.
It looks like you have to calibrate your thermometer first. The easiest way is to use well known termistor (preferably in well working printer) and then measure temperatures with your thermometer. This way will give you proper calibration of it. Then you can measure other thermistors with this thermometer. Of course it requires to keeep conditions constant as far as possible.
But to be honest... I don't really feel (or see) if there is a big difference with temperature +-10C. Let's say my filament has temps from 185C to 225C and I tell you there is no difference (at least I don't see it) if it's 190C or 210C. Of course this difference is crucial when you reach min/max temp but in the middle... :)