On a lot of my softwoods and younger hardwoods, the collar is way less obvious, so I started running my hand up the branch until I feel that little raised swell or wrinkle where it meets the trunk. On maples and elms I wait for that slight bump before I cut, and if it's really flat I'll cheat the cut out an inch or so rather than risk the flush cut. The worst mistake I made early on was on a redbud, cut it flush because it looked "clean" and by year two it had a hollow spot you could stick your thumb into. Now I go by feel more than by sight. Take your time, the extra inch is worth a lot more down the road.