Back off the rocker adjustment so there is a few thousandths play in the valve train. (You may have to do this a couple of times to allow any residual hydraulic pressure to bleed off - if the lifters are hydraulic.)

Rock the rocker back toward the pushrod side and see if the pushrod goes down.
It should go at least .060.
If it does depress this indicates the lifter is hydraulic.
If it does not depress, that would indicate a solid lifter.

If it's a fairly common and unmolested SBC more than likely it's a hydraulic lifter set-up.