I itemized my deduction last year, 2018, in my Fed return. How do I know if my NJ state return also used otemized deduction?

NJ has very few itemized deductions. Look on the NJ return to see what was claimed.

https://www.state.nj.us/treasury/taxation/njit13.shtml

So should I declare my refund in my 2019 or not?

your state refund taxation depends on whether you itemized on the federal return and deducted too much state income tax, not the state return.

I itemized my fed taxes but I thought it also depends if I itemized my NJ state tax and this is what I'm trying to undetstand (I didn't receive a 1099G).

NJ does not mail 1099-G. you have to go to NJ div of taxation website to retrieve it.

just enter your nj refund. TurboTax should calculate how much, if any, of that is taxable.
It may not be taxable.