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.