To verify if the lifters are good, remove them. ( if they come out easily )
It was not mentioned if these are roller lifters ( 96-99 Vortec, 99-02 Classic 2500/3500 ) or hydraulic flat tappet.
If roller, remove the metal spider that holds the dogbones down on the lifter bodies. Grasp, and pull each out one at a time, look at the wheel for any signs of pitting. If none, give the wheel a roll over your hand, if it feels smooth, you will be fine to re-use it. Alternately, you could replace the lifters with OE equivalents.
If hydraulic flat tappet, remove the lifters, making sure you note which position it comes from, an egg carton or other organizer may help you here.
Take two lifters, and put them together on the bottom face, they should 'rock' a slight bit, as the bottom is very slightly convex, ( crowned ) to enable rotation in the bores. If they are flat, or concave, they should be replaced, and it is likely you would want to replace the engine camshaft at the same time. I should note that a flat tappet camshaft has a taper on the lobes, that works with the crown of the lifter. Together they promote lifter rotation.
When one is worn, the other is also likely worn.
Hope that helps.
