Lazy way:
When your car is cold, start it with the AC off, let it idle and warm all the way up. See if your fan ever cuts on. if it doesn't cut on, it could be either your fan or the thermostat switch.

Non-lazy way:
If you just ground the thermostat switch wire, the fan should come on. If you ground it and it doesn't come on, get someone to watch the temperature gauge on the dash while you ground it to make sure you're grounding the right thermostat switch. If the gauge pegs hot, ground the other switch. I forgot which is which...

Fan cuts on: bad thermo switch
Fan sits still: Bad fan/bad wiring.
EDIT: Fan works normally: bad thermostat, bad head gasket or low coolant.

If you have to replace the switch, don't forget to top off your coolant and burp all the air out afterwards.