How do I know that my starter is going out in my manual f150?

 

Um, maybe when the truck doesn't start?

But seriously, what are the symptoms? If it's not turning at all on occasion it's probably the clutch interlock switch.

 

Year, engine, mileage, symptoms...

 

Solenoid is usually the first thing that goes on starters. This leads to clicking noise when turning key. It's simply not engaging when starting. Pretty easy to change solenoid on starter if you're a bit mechanical inclined. Alot cheaper than replacing starter.

 

It's best to remove the starter, then take it to a good parts store that has the correct equipment to "bench" test it. Most do this for free. And while you're at it, have the battery tested too. If its 3 to 4 years old, it may be on its way out, especially if it was a cheap battery that was used as a replacement. A battery begins to drain the day its acid is added. 4 to 5 years is good for a battery; anything over that is bonus time.

If it is found to be weak, be careful when purchasing a new or rebuilt unit; there are plenty of threads/posts on this site where folks have bought rebuilt starters for cheap, only to replace them again in 6 months. Same goes for a battery.

Have you checked your battery cables? Are the battery posts clean? What's the condition/age of the battery cables themselves? The copper wire within the cable can deteriorate. If the cable connectors are green, there's a good chance that the cables are also green inside the plastic insulation, causing poor conductivity, and a great amount of resistance, which ties to lower amperage to the starter.
Check all battery connections at both ends for good clean connections first. Broken or frayed cables need to be replaced.