Only way to be sure is to have it tested. Typically your local Autozone, Pep Boys, Checker, etc. will check both your battery and alternator for free.
A word of advice, you may take your truck to at least 2 of these places for the check. The folks that work at these places are usually not mechanics (if they were mechanics they'd be making more money somewhere else) and may have trouble working even the simplest equipment.
I took my truck to 3 different places. All told me my battery was bad, but only the last 2 places told me my altenator was bad. If I'd just gone with the 1st shop's recomendation to change my battery then I would have had the same problem shortly thereafter.
I think our alternators put out around 80 amps if they are working properly. Ask the guy/gal who tests it how many amps your alternator is putting out. If they look at you like a deer in headlights when you ask this... move on to the next shop
I'm on my 3rd alternator. The factory one died at 40k, my Autozone replacement died at 80k and my 3rd is still in there. I bought the "lifetime" warrentee one from Autozone so I didn't have to pay for the 3rd (or any future) alternators. Luckily that is all I've had to replace on my truck so I feel pretty lucky about that
Good luck.