Ok 1-I would snake the line first to see if it is a rag caught in this drain
2- If no rag or other HARD stoppage is encountered I would use my TRUSTY JetMax Water Jetter and put it on 3,000 PSI @10 GPM to flush away the possible soft stoppage such as silt/sand and or soap scum build up.
Then I would take my MicroCalif Video inspection camera to
1- Double check my work
2- Check the piping integrity
Then I would put your name in my computer for a yearly follow up as I presented you with my bill for the following
1- Snaking the line $125
2- Water Jetting $225
3- Video inspection $500
plus Tax BUT you would have one great flowing drain.