Check square in perspective projection

I'm writing an application which detect squares. I wrote algorithm of quad detection and now I mus开发者_开发百科t select squares of this quads list. How can I do this in perspective projection? Can I transform this 4 coordinates of quad to parallel projection? If I'd able to do this, I can simply check angles and length of edges.

Maybe it solves somehow different? Can you advice me tricks to solve this task.

Thanks!