Our Black & Decker Frymate light went off, but oil is not hot enough. How to check temperature and what can be done?

You did not indicate a model number. Here is a link to to the User and Care Manual for the DF450C:

http://useandcaremanuals.com/pdf/DF450C.pdf

Does your model have an adjustable thermostat; perhaps the setting knob was removed and incorrectly replaced. If it is not this simple you will likely need to find your local customer service centre.

You might consider calling the Black & Decker Customer Care Line specifed on the front of the manual:

1-800-231-9786