Besides what's already been mentioned, the scan hammer will draw waves on the ground in front of you in roughly a 60 degree arc.
After scanning, you can scan and turn again, then repeat until you've done a complete circle.
This will not help you if you're very far away from the max for a given vein, but if you are, and the max isn't in some un-thumpable place (accord restricted zone, rock, etc), a peak will be obvious in the wave pattern.
This is also useful for seeing what areas are thump-able when you're near rocky areas, hills, others people thumping, etc.
When you're not near the max, you generally just scan, move a bit in one direction, scan again and see if it's gone up ... if it has, then keep moving in that direction, scanning every so often until it goes down. Once you find the max in that axis, try going 90 degrees off of it, and see which direction it continues increasing.
Although there are occurances of 80+%, and I've seen a 100%, it's rare to get that high ... you're much more likely to top out in the 50-80% range. (note that this may change with the patch expected this week, as they change around resource distributions)