Minecraft wiki entry for Measuring_distance, associated text: This method is much faster and requires no building, though may not be 100% accurate. After going into the debug screen, the crosshairs are replaced with 3 colored lines. Take the length of the green line (which is always the same if you face near 0 on the Y-Axis) relative to a block. If one normal sized block is the same height as the green line, the player is standing about 35 blocks away from it. If it's 1.5 blocks long 52-53 blocks away, 2 blocks long 70 blocks away etc.
Humans and mobs can also be used for measurement. Pressing F3 + B shows hitboxes. If the green line is as high as a player hitbox, he is about 62 blocks away. With this, observable distances can be measured very quickly. However, aiming upward or downward shrinks the green line and that has to be accounted for if one measure distances far above or below him.