how to spec a cam?

To measure an existing camshaft, I am guessing you mean. You need to either have the cam in the engine and use a dial indicator and degree wheel or you can set it up on v blocks on the bench and measure the lift with a dial indicator that way. It's pretty easy make a good guess at duration from the lift, but the degree wheel will take any guesswork out.

Or you can send it to Oregon Cam and they can map it for you in the cam doctor.