Im looking for a way to detect if the device is using retina display, or to figure out its true resolution size, or if i can detect if the device is using one of my 'imageSuffix' sizes.

For image suffix im using '1.5' and '2'. and my 'application -> content -> width / height' is 320 / 480.

I need a value in return for my condition.

Any ideas?

Have can have a look here:
http://developer.anscamobile.com/reference/index/displaycontentscalex

From this you can calculate the actual resolution of the device your app is running on.