[leebca]

There is basically no way to determine if a tank has been exposed to a copper medication/treatment without performing an analysis. This would require a micro analytical procedure, as far as I know. I don't know of any 'home test' method for this.

There may be some clues. Sometimes, if it is an all glass aquarium, the sealent may be discolored. But this can be from a variety of different reasons, so it isn't a guarantee. If the sealant is blue in your tank, I'd first suspect the use of Methylene Blue rather than copper. If the sealant is green, I would suspect copper. But as I wrote already, there is no guarantee to this method AND there is no way of telling by how dark the color may be.

If in doubt and you still need or want to use it, then clean it as if it had been exposed to copper meds. On the other hand, if you don't plan on putting invertebrates or live rock in the tank, then it should be good to go for a FO marine tank.

To your edit questions:
"Wash" is meant to convey more than just a 'rinse.' Use a clean/virgin sponge or clean/virgin rag to rub the walls where 'wash' is mentioned.

No more time is needed then just the wiping with the sponge or rag. All surface areas must be wiped-rubbed with the sponge or rag.