I can give you two places where you can see the skyline, from the furthest distance I know it can be seen and only on a clear day:

1) The PA-NJ Turnpike connector bridge over the Delaware River at Bristol. The skyline is at your 10 o'clock when coming back to PA (westbound). I noticed this for the first time yesterday.
2) Baltimore Pike in Springfield. Travelling east from I-476, you crest a few hills between the mall and when it goes down to one lane in Clifton Heights, and some of these have an unobstructed view of the skyline. Look to your 1 o'clock.