Is there a rule for when contractions are not possible? [duplicate]
All OP's examples are cases where the verb is stressed. In many/most contexts, this applies where the verb ends the statement (#1 can be recast as Can you tell by my voice alone who I am?).
The most general "rule" I can think of is don't contract anything unless you hear native speakers doing it. Also bear in mind that contractions primarily reflect spoken usage, but many aspects of speech aren't necessarily reflected in written usage anyway. So perhaps the second "rule" should be don't write a contracted form unless you see native speakers using that written form.