A retained eyecap will usually not be completely clear. It may look a little thick. Have a dent or dents. Wrinkles or lines going through it. Anything other than a clear glass-like eye.
Just remember that even if the eye caps came off you may see one or more of these things on the eye if the humidity drops too low. Dont automatically assume that its always a retained eye cap. If thats the case the dents or wrinkles will go away within a day or 2 if you raise the humidity.