Because of reciprocity, your W-2 income is taxable only by your resident state of NJ. If your employer withheld NJ taxes, it would be reported in Box 17 of your W-2 on a line marked for NJ.
If they mistakenly withheld PA taxes, there would be an amount in Box 17 on a line marked for PA.
If there are no amounts shown in Box 17, then no state taxes were withheld. Even if no NJ taxes were withheld, you still have to file a NJ tax return if your income exceeded NJ's filing threshold, which you can find here:
https://www.nj.gov/treasury/taxation/git_over.shtml
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
