Yes. A biologist must determine whether the trait under investigation is heritable. This can be determined experimentally for quantitative traits.
No. A biologist must demonstrate that more offspring are produced than can survive.
No. A biologist cannot verify postulate 2 of evolution by natural selection (variation is heritable) without determining the alleles responsible for variation or the molecular mechanism of adaptation.
Yes. However, a biologist must identify the DNA sequence of the gene or mutant amino acid in the protein responsible for the adaptation.