How do I know which part of the already withheld tax abroad I can offset against my tax payable in the Netherlands? Does this also apply if I am fiscally resident abroad?

With the annual statement, you can calculate which part of the already withheld tax abroad can be offset against your tax payable in the Netherlands. You do not have to pay double tax.

Does this also apply if I am fiscally resident abroad?

You should ask your tax advisor this question, as it may vary by country.