Summarize:

While this method is less accurate, you can use it when the shadow method won't work, such as on an overcast day. Also, if you have a tape measure with you, you can avoid having to do math. Otherwise, you'll need to find a tape measure later and do some simple multiplication problems. For the most accurate measurement, you should stand so that you are on a piece of ground that is about level with the ground at the tree’s base, not higher or lower. Your view of the tree should be as unobstructed as possible. You can use any small, straight object, such as a paint stick or ruler. Hold it in one hand and stretch your arm out so that the pencil is at arm’s length in front of you (between you and the tree). That is, your friend’s feet should be aligned with the pencil’s tip. He or she should be about the same distance from you as the tree is, not further away or closer toward you. Since, depending on the height of the tree, you may need to be some distance away from your friend, consider using hand signals (with the hand that is not holding the pencil) to tell him or her to go farther, come closer, or move to the left or right. You'll need to measure the length of each mark and the height of your friend, but you can do this after you go home, without having to return to the tree. Scale the difference in lengths on the pencil up to your friend's height. For instance, if the mark showing your friend's height is 2 inches (5 cm) from the tip and the mark for the tree's height is 7 inches (17.5 cm), then the tree is 3.5 times as tall as your friend, since 7 inches / 2 inches = 3.5 (17.5 cm / 5 cm = 3.5). If your friend is 6 feet (180 cm) tall, the tree is 6 x 3.5 = 21 feet tall (180 cm x 3.5 = 630 cm).  Note: If you do have a tape measure with you when you're near the tree, you do not need to do any calculations. Read the step above for "if you have a tape measure" carefully.
Use this method as an alternative to the shadow method. Stand far enough from the tree so you can view the whole tree—top to bottom—without moving your head. Hold a pencil at arm's length. Have your friend move so that you can sight him or her “through” the point of your pencil. Find the answer once you have access to a tape measure.