The deductions for your fees are automatically stated in your tax assessment notice. SKAT.dk will automatically receive the information of how much you have paid in fees for 2017 from your A-kasse.
However for 2017 you can check that it gets automatically stated - you can do this at SKAT.dk when the tax assessments for 2017 are ready, this will be about mid marts 2018.
When you are logged into SKAT.dk you must look after the document called "Årsopgørelsen 2017".
If it is NOT already stated there, then you can deduct it yourself online at SKAT.dk.
You are entitled to deduct all paid unemployment insurance fees. There is no maximum limit.
However for Trade Union you are allowed to deduct a maximum of DKK 6.000 in fees per year.
You can read more in english about deduction for Union and Unemployment Insurance fees at SKAT.dk:
Best regards, and happy new year!