Determining the EXACT declination

All you really need is your map and your compass.

The Declination on Maps is Approximate as are most Declination Finding Utilities.

To begin, determine your exact location on your map. Now find a landmark on the map that you can visually and easily see from where you are. Obviously, the further away that object is, the better. A mountain peak, a microwave tower, a point of land jutting out into the lake are all perfect.

Take the magnetic bearing to that object. Next take the bearing on your map. The difference is the true, real, up-to-date declination. It's that simple! If possible, take more than one landmark to confirm your first answer.

If there are no suitable targets, wait until after dark. You can then use Polaris, our favorite North Star, as due North. The most you can be off is about 3/4 of one degree. On July 1st Polaris is exactly North, so the closer in date to July 1st, the less the amount of error. It's not until the 1st of December that Polaris is a full 3/4 degree away from due North.
In practical terms, this means that if you're canoeing in early August and using Polaris as your determination point, your declination error would be 1/6 of 3/4 of 1 degree. In real terms, that error rate is way too minisule to be meaningul.