How can you tell if your fuel gauge is bad on 1997 blazer

Likely, it's the sender unit in the tank. To check wiring and your dash gauge, you can pull apart the connector at the fuel tank, find the wire for the gauge sender unit (its color coded-I dont kinow what the one for your car would be), and on the harness side(the one going to front of vehicle) use a jumper wire and ground that wire to the car frame(get me? you want to ground the wire going to your dash gauge)-with key on, your gauge working properly will move up to half-way or even full. That tells you your dash gauge is working and your wiring to gauge is good. If gauge doesn't move when grounded(double check for a good ground), then your dash gauge may be bad.

USSUALLY WHEN GAGE GOES WAY OVERFULL IT'S A BAD GROUND
QUICK CHECK USE BOOSTER CABLE ONE END ON TANK OTHER END ON BATTERY NEGATIVE IF GAGE GOES OK MAKE UP SHORT WIRE FROM FRAME TO TANK SEEM LIP
AND ATTACH MAKE SURE METAL IS CLEAN

Is this the only gauge on the vehicles instrument panel(cluster) that doesn't work? Check fuses. May need to check this from tank unit wire harness by gas tank for bad gauge or defective tank unit. Good luck, best regards.

I would guess that your sending unit from the tank is grounding out which means it need to be replaced in most cases. If the gauge is bad, it will usually effect the other readings in the cluster.

Was the wire harness replaced from the fuel sending unit to the frame, it's a good practice to replace the wire harness when the sending unit is replaced. The wires are know to break and cause sending unit issues from weak fuel pressure to faulty gas gauges.
The other possibility would be a faulty connection to the stepper motor in the gauge it self or just a faulty stepper motor that would need to be replaced. You can do a search on Ebay for stepper motors for your fuel gauge which runs for around $15.00 which is much better then $400.00 for a new cluster if it is and bad fuel gauge stepper motor.
Thank you for using fixya and good luck.

The pump won't work if you unplug it, I'm just trying to help you diagnose the gauge. If unplugging it doesn't make the gauge go the other way then hook a jumper wire between one of the black wires and the purple wire. Turn the key on and see if the gauge moves the other way. If it moves when you ground it test both black wires for good ground. If they are good, replace the sending unit in the tank, again, even if its new, the parts store should warranty it if its less than a year old.

Unplug the fuel pump, turn on the ignition and see if it goes to empty. Is this a 4 or 2 door blazer? If it is a 4 door this will be easy.

Gauge pegged at full or empty?

THE FUEL GAUGE IS FAULTY

VERY COMMON GM problem . the sending unit in the tank is faulty. You are usually better off replacing the whole fuel pump module assy as opposed to only the sender because you can damn near buy the whole assy cheaper than you can buy the sender only for at the dealer which is where you will have to buy the sender if you only want to replace it and not the whole "She-bang".