The past couple of weeks, the A/C in my Toyota Corolla (approximately 92,000 miles) has fluctuated from blowing cold a/c to blowing hot air. There doesn't seem to be any leaks; how do I check freon levels? Add more if needed? If the freon level is fine, and the relay switch is OK, what else could it be? I'm in Texas and a/c is a necessity! :) Thanks!

Go to the service valve on the smaller (high side) pipe. Unscrew the cap, stand back, and press a small screwdriver tip down on the shrader valve quickly.
A quick blast of refrigerant should come out. You should be blowing cold if it does. If you get the blast, and still not cold, then update your clutch relay from a 02022, to a #02028. That's what the TSB says for your year.
If you get nothing out of your pressure test, you have lost all your refigerant. I'd be checking the opening in your front bumper from below. You may have taken a rock into the condenser. Look for oily spots.
What really should be done here is a qualified A/C technician with proper equipment to diagnose the A/C of your vehicle. We go through intensive training and qualifications for this service.
These are the TSB's that come up for your vehicle.

AC009-04: Sensor Inspection for Air Conditioning System: 1989 - Current All Models
2. EL011-05: A/C Light Flashing--Magnetic Clutch Relay: 01-04 Applicable Models
3. AC003-04: A/C Compressor Installation Procedure: Applicable Models
4. AC002-05: 03-05 Corolla: Heater Performance Improvement
5. AC002-03: 03 Corolla & Matrix: No Air Flow From Center Vents
6. AC002-02: 03 Corolla & Matrix: Low Air Volume From A/C System