How do I tell if my Thermistors are 10k or 100k?
As @DwayneReid already said: take a multimeter and measure the resistivity. It will either be in the ballpark of 10KOhms or 100KOhms. If you already got a readout close to room temperature you're resistivity put in the firmware should be fine.

However, now it is still important to find out which make of thermistor you have, since they all behave slightly different. Find out about that in the manual of you 3D printer / hotend or in the documentation of wherever you got the thermistor from. Select the corresponding entry in Marlin then.