One way to "see" wind puffs is to look for ripples on the water surface. Another good way is to watch other boats that are ahead, behind and near you, regardless of whether they are racing or just daysailing, and see what is happening to them. If you see their sails fill and they start to heel more, you know they''re in a puff. Recently, I was running downwind in a dying puff a few hundred yards from the finish line, and looked to my left and saw a competitor with his sails straining in a new, strong puff, so I broad reached over and got into the leading edge of the puff, turned downwind wing-and-wing and roared down to the finish line ahead of the other boat. If I had stayed where I was, the other boat would have beaten me to the finish line. Look at flags and trees near the shore, and anything else that gives you a clue as to what the wind is doing. There are indicators out there that can tell you what the wind is doing, and the more you look, the more you see. At one time, I lit a cigar and watched the smoke rise from its tip to see which way the air was moving in ultra-light air. (I quit, however, because I started enjoying the occasional puffs that I had to take on the cigar to keep it lit.)
It sounds like you are making the right sail choice. When you get around to replacing the mainsail, consider the shelf foot and flattening reef. They really help in light air.