Hard to get in gear intermittently

[DT_Duh_Trucker]

My 14 prostar randomly is hard to put in gear from a dead stop sometimes not all the time. Mechanic said on my truck there is a self adjusting clutch so he can't adjust it.He said he checked it and it's the clutch. Is this a for sure it's the clutch or is there something else I can consider before dumping money into the clutch replacement? Just want to make sure that's the problem and we're not just throwing guesses at the truck. How would you go about verifying it is the clutch?

Thanks