As a one time thing to determine declination, that can work too. While the GPS internal electronic compass is a magnetic compass, it has a declination table and allows you to select between true North and Magnetic North display. Easier would be to just look at what declination it is using when you select magnetic. It's easy to forget to turn the electronic compass off and then you lose battery life. Another nice thing though is that it's easy to carry a few spare AAA batteries.

Geez, it's been a while since I used that function, forgot a lot and had to turn it on and play with it a bit, going through all the menu options. It's good to be at least minimally familiar with your gear.