To check the detents, just pull the feedneck off by unscrewing the screws that attach the feedneck to the body. You can inspect it that way. Another way is to ***make sure the co2 tank is off and no air is in the marker*** is to cock the marker back so the bolt isn't against the detents and see if they are still there.
They tend to break off. I know smiley was waiting on Lance to get him some, I don't know if he has any to spare. But I do know Palmers sells them here: VM68 / PMI 3 Ball Retainer [VM68028] - $9.45 : Palmers Pursuit Shop, Where Custom Still Means Something