If there is a sewer smell in your basement, but your sewer appears to be fine and not backing up, simply try flushing the toilet regularly. You should also check your "P" trap (basement floor drain) and make sure it has water in it. If not, add water to the trap.
If you notice a smell during the winter months, check your roof vent to make sure it is not plugged with snow or ice. Once the vent is cleared the smell should dissipate.