Stock 93 240sx is running hot on hot days, or after >25 minutes driving. No apparent leaks. I have replaced the thermostat with a new Nissan thermostat, flushed cooling system, bleed it, and pressure washed the radiator. Before I replace the water pump I would like to check the fan clutch. They both turn on and spin, but I have heard that the clutch might still be bad. How do you check it? Also, any other possible causes?

Yes, the fan clutch can cause this problem. But don't forget the rad. cap. If not holding pressure,the coolant system will run hot. Also if the coolant mixture is more than 50/50 coolant to water, to much antifreeze, the engine will run hot. There is also the chance that the radiator cooling tubes could be clogged internally and causing coolant restriction.

Could be hoses, but could also be more than that or less. Fan clutches are not a big deal. Most people who want easy and quick upgrades usually start by completely removing the fan clutch. I have and i dont need it. The electric fan will cut on when the engine begins to get hot anyway so intern still keeps it cool. Make sure that the electric fan is kicking on also. you can tell if the fan clutch is good or bad by seeing if it will turn with the motor turned off. If it turns easily then its good. if it just sits there and keeps spinning then its bad or if it wont turn at all, then its bad. It should only turn 2-4 times when its good, if that, with one turn of the hand. you can also check the post i left on another question about "burping".