Run a cranking compression test. If your hydraulic lifter is stuck, it should have a low pressure on one cylinder, as it is keeping a valve open. Should you find that it does have a bad lifter, its probably easier just to change them all. Their not that expensive if your running stock, and its never fun to have to replace them 3 months after you replaced this one. Be sure to pull the distributor wire while you are testing, and i would reccomend removing the fuel line and placing a collector. Hope this helps