Your best bet is to:
1. Consult your owner's manual: If you still have it, the manual should have a diagram showing the location of relays and fuses.
2. Check the underhood fuse box: This is the most likely location. The fuse box usually has a cover with a diagram showing what each fuse and relay controls. Look for a relay labeled "cooling fan," "radiator fan," or something similar. Sometimes they are not labeled clearly and require tracing the wiring.
3. Trace the wiring from the fan motor: Follow the wiring harness from the coolant fan motor itself. It will lead you to the relay.
4. Use a wiring diagram: A wiring diagram specific to your 1988 Chevy Celebrity 2.8L is your best resource for definitive placement. You can find these online through auto parts websites (like Advance Auto, AutoZone, etc.) or via online forums dedicated to classic Chevy vehicles. You may need your VIN to ensure you have the correct diagram.
Remember to disconnect the negative battery cable before working on any electrical components to prevent accidental short circuits.