If battery is weak put it on charger less than hr truck cranks now wondering if it is alternater
If it now cranks and runs normally. Test alternator it should be putting out around 14.5 volts or so.
If I start truck can I pull battery cable to check to see if alternater is bad
No don't do that. Can cause damage to diodes and ECU. Have it tested at AutoZone, O'Reilleys someplace like that..usually they will test free.
What truck are we talking about here?
Yes please don't do that. Old school testing does not have good results on newer vehicles. Can cause expensive damage. Go to auto zone and have them test it. Only takes a minute or two.