Check a control

[Lupo73]

I'm using nsDialogs.. I have created a custom page where the user can load a file (with SelectFileDialog) and now I need to make a check, when the user click Next, to verify if the file selected is valid or not. If is not valid (if the name of the file doesn't contain the string "Pack") I need to open a message that ask to the user to select a correct file. Thanks for help!