85 Chevy C30 a/c flush question

[prateroad]

Just flushed evaporator and condenser with auto parts place flush. Blown out with clean air. How do you know if you got all the flush out? I googled a lot but no decisive answer. I don't want to leave it open for too long for the stuff to evaporate and let moist air in. I am going to flush with nitrogen before closing up for the vacuum. Will the vacuum pull out any flush residual? Any help appreciated. Installing new compressor also.