In Terminator, Kyle Reese says only living tissue can go through the time portal. But he also says that they found the time machine after the T-800 had gone through and had to quickly send someone after it before blowing the place up.
Where did the Resistance get so much info on the machine? Did they know about it beforehand? Did SkyNet leave the manual lying around? Was it a guess after watching Arnie streak across the room?