Determine if user has pop-up blocker on Flex
I'd use Flex's External interface in conjunction with one of the JavaScript answers.
I would use javascript to check for popup blockers and pass the result to Flex. Here's a similar question that may help.
To tell you the truth, I am not a Flex expert, by any means, but if you are not using Flex's built-in dialog tools, I would recommend using jQueryUI. You can do anything from simple alerts, to complex HTML forms, and even externally loaded pages, if you want.
The jQueryUI has their own built-in dialog tools, which does not trip the browser's pop-up blocker.