Fair question.
Classic method would involve measuring boiling de-ionised water at sea level to determine 100deg C, but max range quoted as 70deg. Similarly, iced water at 0deg, checked mix at 15 min intervals and determined as mean. No time to faf about like that!
So, compared with my quality thermostat for the gas fire, reading 20.8deg C- within 1 degree.
The "outside " probe placed within armpit for five minutes registered , after 1 minute, a steady 34.7 deg. C
The declared accuracy reads" range outside -50deg C to +70 deg, tolerance -50-0deg c +/-2deg, 0-30deg, tolerance+/- 1 deg C, 30-70deg C, +/- 2 deg C"
Close enough for our purposes I think.