Like you're talking, prior to August, it's a trick to estimate where a buck will finish out. I think this is where having seen huge numbers of bucks over the years comes in handy. Experience with having seen many bucks in July and then again later is key. That, and also being able to really visualize finished out bucks and comparing that to what you're looking at in July.
Comparing the thickness of the ends of tines compared to the overall thickness of the bucks lower end helps. No doubt, it's a trick and takes experience to try and hone that skill. I'm a long way off from mastering that!
Any scouting trips I do in June are mostly in hopes of spotting a buck that has special width, or maybe huge eyeguards, a droptine, or maybe a webbed out bottom end. Often times those scouting trips have not been very productive. I do believe however I've seen a good number of bucks in late June or early July that I never did go back and look at that probably ended up real big.
I've also found that video is extremely important. I don't know how many bucks I've looked at on the hill that I thought was a certain caliber, only to return home and have the video bring me back down. Video helps to keep my mind from playing tricks on me. And it allows me to really analyze a buck over and over, comparing antler lengths to ears, heads and tails. That helps a lot in analyzing June and July bucks and making that guess on whether they're worth going back to look at.
As far as when bucks finish out, it's all over the board. Some are done in late July, others can put on 4-5 inches in August. Trying to hone skills on judging size, when they'll finish out, how will they finish out, etc. is what keeps me going back.