For me, it's not about "knowing" so much as "this player has the type of floor that could put him out of baseball in 3-4 years". Arcia doesn't have faults, he has some glaring issues that need to be resolved for him to be a long-term fixture at a defensive position. If he doesn't fix one or more of them, he's a platoon bat. Platoon bats don't get extensions. The day Byron Buxton hits Minnesota, I won't know if he'll be a 90 OPS+ player or a 140 OPS+ player... But I'd pursue an extension anyway because Buxton, as an up-the-middle defender, can be of real value whether he hits like Babe Ruth or Nick Punto. Sano, while not a defensive force, should hit enough to be a DH no matter what happens so an extension should be explored there as well. Arcia hasn't shown that he can even hit enough to hold down DH. It's about risk mitigation. Is the floor of this player so low that he could be out of baseball before the contract is done? If yes, then I drag my feet on an extension until that player shows me he can hold a place on an MLB roster.