Our basement has occasional sewer smell. There is a full bath in basement but is used very little. How can we track down where the smell is coming from? Since we did not install bathroom I'm not sure if it was done correctly.

Sniff around to where the smell's the worst. In basements it's generally the floor drain that gets overlooked. If this is a unused drain pour a quart of cooking oil down the drain for a reap seal that won't evaporate.
Good luck, Tom