Some methods follow. The limb tips of your bow can be used as a framing reference. For instance, a target of known size will appear to be larger at closer distance and smaller at longer distance in relation to the size of your limb tip. Also, you can frame the target between the wheel of the compound bow and the limb tip to get a reference of distance. You can use your sight adjustment knob or virtually any specific sized part of your bow or sight for this framing method.
Some archers also hold their arm fully extended as if pointing at the target.
Some archers have been observed doing this with the thumb ring of their finger sling dangling below their hand. The key to this is to find a reference on your bow and practice looking at targets, 3-D or other, of known size at known distances in relation to your chosen framing reference. 5 yard or 5 meter intervals are easy to detect with this technique. Another point to remember is that the framing reference must be kept a specific distance from your eye in order for this to work. With a scope and a target of known size, the following method should be employed.