I've come across solids that have the retainer clip. The body of the lifter has a shoulder where the top fits against and is held in by the clip. Short of taking the lifter apart to see whether or not it has a plunger and check valve or just a simple counter bore for the cap I'd suggest this. Stand the lifter upright on a firm flat surface and using a pushrod try to push down on the top cap. A solid will not budge. A hydraulic will give and depress somewhat depending on if there is any fluid in it or not. You should be able to easily depress a hydraulic lifter between 1/16 to 1/8 of an inch with just moderate pressure.
If there is any doubt about the cam call the outfit you bought it from and give them the part number on the cam. Don't mix cams and lifters.
