there are 3 common methods
1. visualization
2. increments (5-10-20 yard increments)
3. half way to target
1. some people just look at the target, esp in asa. they use the same targets over and over at every shoot. you will have a mental guess when you first walk up to a target, wether you mean to or not.
2. some guess in increments, 5-10-15-20-25-30-35 then break down to 1's
some do 10-20-30-40 then 1's and some do 20-40 then 1's
3. say i know half is 22 yards, so its 44 yards
i use #1 and #2 methods. i will say 1 is right more than 2. #2 i use 10 yard increment and 20 depending on terrain. plus ive gotten better after i left my rangefinder at home on ranges (not tournaments) and depended to know how far it was/is by my judging.