How do i tell if the fan clutch is bad?

Answer 1

Car stays cool at idle and slow speeds...overheats at faster ones.

--I would actually say the opposite. At faster speeds, more air would be flowing through the radiator, causing cooler temps. In slow traffic around town, you would see higher temps because your fan isn't spinning properly, and you don't have the benefit of air flowing through your radiator like you would at higher speeds.

What I know the fan won't stop when you turn of the motor.suerte

No, a bad fan clutch, as per the original, causes overheat at speed, and works fine at rest.

The clutch disconnects the fan from the pulleys at speed, so it free flows and allows air to pass through easily. When bad, the fan stays engaged, which can't spin fast enough to through air at say 50 mph, essentially acts as a BLOCK to the air, and car overheats.

That is actually just one theory of a bad clutch. yes if the clutch does not release at high speeds it will act as a block which is a stuck clutch which is only one of two clutch issues. Another clutch problem is when the clutch does not activate at idle or stop and go traffic thus not pushing air into the radiator causing a overheat. So yes you can have clutch malfunction at 50 mph and also at idle or stop and go traffic.