Place a small amount of granulated sugar in an oven-proof dish or on a cookie sheet lined with aluminum foil.
Place in oven for 15 minutes.
If your oven is calibrated correctly, the sugar will melt.
If your oven is running cold, the sugar will not melt.
To test if your oven is running hot:
Preheat oven to 350°. (177° C.).
Follow the same procedure as above.
If your oven is calibrated correctly, the sugar will not melt (although it may brown a little).
If your oven is running hot, the sugar will melt.
Note: ovens do not maintain a constant temperature, but cycle above and below it, so it is possible that the sugar may melt at 350° if your oven is correctly calibrated but has an extreme cycle (15° F, 8° C.).