Calibrating a thermometer is not super easy but here are some potential ways.
Bowl of ice and water, well mixed, should be exactly 32F
Actively boiling water should be 212F
If there is any way to expose your sensor to either of those, you should be able to see how calibrated you are.
Alternatively, you can purchase an RTD based sensor as a reference, those can be calibrated at the factory to be a little more accurate.
The infrared "gun" type temperature sensors are mostly garbage. They will give different readings of different materials even if they are all the same material, because different materials emit light (in this case, infrared) differently. This is called emissivity, and some temp guns allow you to adjust emissivity, but then you need to refer to literature for the emissivity of a material, and so on.