The fossil was discovered.

I determine whether this is a COPULA VERB + ADJECTIVE or a VERB TO BE + PAST PARTICIPLE (PASSIVE VOICE) by determining if there is a clear agent that could be the doer of the verb to discover.

I feel this is a predicate adjective. Agree? How do you test?

Thanks

The fossil was discovered.

One of the best ways to determine whether a past participle is part of a passive-voice verb or an adjective is to recast the sentence and see whether the participle makes sense as an adjective.

The vase was stained. It was a stained vase. [yes]
The window was broken. It was a broken window. [yes]
The soldier was promoted.  He was a promoted soldier. [no]
The fossil was discovered. It was a discovered fossil [I would say no]

My feeling is that the sentence is in passive voice with the discoverer not named. This is not 100-percent effective, but helpful.