To begin with I can't understand why you would expect someone to diagnose your problem for free.

The evaporator is indeed a trouble spot on these cars. If the system were charged and UV dye added, you could shine a black light in the condensation water that drips under the car to check for the dye. If there, it is the evap. Since it is apparantly not charged, this would not be practical since it is an R12 system.

The alternative would be a savvy shop could add two ounces or R22 and pressurize the system with nitrogen and then check the evaporator with an electronic sniffer. This would be tough, but could be done.

R12 is still available to EPA licensed techs and is still the preferable refrigerant for your car. These cars have very little reserve capacity which makes them bad candidates for conversion to 134.

There will be those who will propose all sorts of snake oil refrigerants like Freeze 12 or Dura Cool. All of these snake oils fall into one of two categories, they are either flammable or a blend. You don't want to use either.

If it is indeed the evap, it is a BIG job to replace. About 15 hours. This will equate to a job that will cost something like $1500 to $2500. Don't panic until you have diagnosed first. It could very well be that you are simply facing another leak. Your indy possibly just figures it's the evap since that is a somewhat common failure. Maybe he is preparing you for the worst.

Did he actually do diagnosis or did he just say that this was probably it?

Good luck,