We could get really complicated with this. But basically, magnetic north is a mostly fixed place on the surface of the earth. It's actually moving, but very slowly.
Sometimes the map you have may have two compass circles on it. One inside the other. The inner circle identifies magnetic north, and the outer circle identified true north. In addition, the map may also note the mean magnetic declination for the map. That being, for this drawn map you can consider magnetic north to deviate from true north by X number of degrees.
If your map does not have this, then you can do something else at night. Hang a piece of string with a weight at the end, and find the North Star. I will go ahead and assume everyone here knows how to use the big and little dippers to find Polaris (aka the North Star). Site the North Star through this string. Draw a line (or use a straight edge if you have one) from the plumb string on the ground so that it points to the star. Now put your compass down on this line so that N lines up with the North Star. Mark your compass on the spot where the needle is pointing.
Now so long as you don't travel very far if you use your compass to find north, you can look for either one. If the needle is pointing at the N symbol on your compass then your compass is pointing to mag north. If you make the needle point at the mark you made, then N is pointing to true north.