My trane airhandler is not draining condensation.The water is backing up in the return duct.. The pvc drains are clear,but the water is being blocked.. How do I check the inside of the unit for a blockage?

Sometimes you can stick a wet vac onto the end of condensation drain with rags stuffed tightly in and around the vac. Hose end ,and suck out whatever has the drain plugged up