For freeze condensing you need to have beer that has just begun to freeze so you can skim off the ice crystals which should be nearly free of alcohol so that what is left has most of the alcohol left in it. If you are careful and repeat the process, you can increase the alcohol percentage by quite a bit. Starting with frozen beer may be more difficult. You might be able to separate the alcohol as it thaws though. Try collecting the first runoff to melt and test the gravity with your hydrometer. If it is lower than the FG when kegged, you have been successful.