I don't know if this is the "correct" way, but I took my alternator up to AutoZone and they can run the appropriate tests to determine charging output to the battery - there's no charge for the test. The two potential alternators that were used on my 2001 F-150 have different body thicknesses - that's how we determined which alternator I had. On the other hand, here's a simple test to check your alternator before removal from the truck: With the truck engine off, check the battery voltage - should be at least 12.6 VDC. Start the engine and recheck - the voltage reading now should be 13.6+ VDC. This would indicate the battery is being charged by the alternator and that the voltage regulator is "regulating". There are also a ton of videos on YouTube about testing your battery and alternator.

 

Well, you can just look at it.
The casings are different.

I could describe it
But.....navigate to rockauto, look at pics of pulley side
You will figure it out.

Hint....look at holes within the gussets.
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Alternators dont store charge, they are rated in current only, amps.

 

what truck do you have

 

My truck has the towing package, so it came with the 130 amp alternator. If you can determine that, i.e. factory receiver hitch and 4 pin trailer plug ...

 

Here is how to differentiate the 95A and 130A alternators.

 

Here you go
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