How's your Spanish? Normally, you should be able to find a website from the national Labor Department, or the National Health Service that will direct you to a listing of the payroll deductions that are obligatory for employers. But these sorts of websites are normally only available in the local language.

You may also want to try the EU website (europa.eu) which has lots of information on working in the various countries available in multiple languages. It takes a bit of digging around, but there's a real wealth of information there.

In most of the EU, national health insurance is deducted from your paycheck (with the employer kicking in his share) and it's figured as a percentage of your gross pay. It's normal for the deduction to cover you and any dependents, but if they have a private health insurance (sometimes to cover what the national health doesn't), that will have a per-person-covered fee.
Cheers,
Bev