Can a radiologist tell if a lung nodule is benign by just looking at it?

Often but not perfec. Often there are certain characteristics on the xray that are helpful but no guarantee. It is often helpful to obtain a pet/ct scan as it has more accuracy. The best test is a biopsy if possible.
Not exactly. There are common characteristics of malignancy or benignity so they make the most likely educated guess. If ther are multiple x-rays over time, their accuracy is enhanced. Hamartomas, and granulomas have rather specific patterns and the accuracy is high. Ask for some details.