LCD rated at max 5.5 V used at 6V
You can't tell whether you fried it, that would be proving a negative.  However, you should assume you fried it, especially since it doesn't appear to work anymore.  You violated a absolute maximum spec, so really shouldn't be surprised.  Violating such a spec is no guarantee of damage, but you can no longer rely on any other specs.  Even if a over-stressed unit appears to work, it might be damaged in subtle ways and fail sometimes in some regions of its operating range.

You screwed up.  Toss out the damaged device, learn the lesson, and move on.  Trying to get anything useful from this device will only result in lots of frustration and probably no useful result.