how to check if starter has a good ground?

starter whirls when taken out and hooked to external battery. when its in only the relay closes. its got power to the primary and the solenoid. and its a brand new starter. gotta be the ground? theres no fuses or safteys in the primary power supply line. i herd the pcm grounds the starter? there is no ground wire it grounds via the casing on the engine block. im goin nuts tryin to figure this out.my dash lights and all work its an 87 dodge dakota 2.2 no turbo, manual tranny. the motor turns if i pop it. just clicks no starter engaging.

There are 2 parts to continuity: Resistance in Ohms and Amps. For example, any fuse has 0 Ohms resistance, but a limit on Amps.
With the Positive Cable removed from the battery- Put Meter Leads at the Negative battery post(not on the cable) and on the case of the Starter. You should read less than 1 Ohm. Do whatever replacements are necessary to read less than 1 Ohm. That only tells you that somewhere on the Starter Case's there is contact with the engine. That contact may have only 1 amp capacity and is destroyed (like a fuse) when you try to start the engine- so no start.
Consider replacing all the thick (high Amps) Negative and Positive Cables you have. Sometimes, the only test is to swap out a part and see if things improve.
The next step is preping the surfaces. Remove the Starter from the engine. Using Electronics Cleaner Spray and a brass metal brush- scrub the terminals of all wires that attach to the Starter. Using Brake Parts Cleaner Spray and a battery terminal cleaner brush- scrub the bolt threads & nuts. Use steel brush on body of engine that comes in contact with the Starter. Use a file on the engine to get new metal exposed to the Starter. Make sure the mating surface is flat and shiny. Use Electronics Cleaner Spray for final rinse to clean the surfaces.