For some reason food.com ran this suggestion last year on checking oven temperature without a thermometer, and it showed up on the inter-tubes today.

Ingredients:

granulated sugar
aluminum foil (optional)

Directions:
1? To test if your oven is running cold:.
2? Preheat oven to 375°. (186° C.).
3? Place a small amount of granulated sugar in an oven-proof dish or on a cookie sheet lined with aluminum foil.
4? Place in oven for 15 minutes.
5 ?If your oven is calibrated correctly, the sugar will melt.
6? If your oven is running cold, the sugar will not melt.

7? To test if your oven is running hot:.
8 ?Preheat oven to 350°. (177° C.).
9? Follow the same procedure as above.
10 ?If your oven is calibrated correctly, the sugar will not melt (although it may brown a little).
11 ?If your oven is running hot, the sugar will melt.
12? 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.).

Use a thermometer.