How can one know which portable external HDD cán be turned into an internal HDD ("shucked")?

Solution 1:

Not every.

It depends really on the design of the disk. Some may have SATA port, or some may have different electronics between port and the disk inside.

Mostly many of them will be SATA but you are not sure unless opening the disk (and having void warranty) and checking, and connecting it would be a sure situation as this may be modified SATA.

So unless you open the disk, connect it and check the data on disk - you are unsure.

The thickness is the second parameter which can block you from putting the disk into a laptop.

Also laptop can have or will not have desired space for each drive's thickness.