If it is a 15-90 or 15-91, they are very good machines well worth getting. The serial # will be on top of the base in the front right corner.
If by some long shot it is a Singer 15NL or 15CH, take a pass on it. These were later made "nostalgia" machines made in Taiwan and China and have a poor reputation. The model and serial #s for these will be on the under side of the cast base.