Can a radiologist tell if a nodule is cancer by looking a on a digital mammogram?

No: The only way to be sure is to get a biopsy.

May suspect: The radiologist uses her/his best judgement to cathegorize a mammographic finding. The findings are given a cathegory number from 0 to 5. 0 means additional testing is needed. 1 is negative. 2 is benign. 3 is probably benign but requires a shorter term (6 month) recheck. 4 means suspicious and 5 means highly suspicious. A cancer diagnosis is made only by pathology exam of a biopsy sample.

Probability of Ca: Nodules are characterized on mammo by various features that increase or decrease the probability of cancer. In order to remain accredited to interpret mammo, radiologists are required to render a final interpretation categorizing the lesion into one of the following categories: benign, probably benign, suspicious, or highly suspicious. The last category implies at least a 95% chance of malignancy