Take someone emotionally uninvolved with you to see the puppy.
Take someone who has no stake in the decision with you to see the puppy. Take a sibling, a parent or a friend. Give them veto power. If they say no to your buying that puppy, back away from the purchase. Take them aside and listen to why they are uncomfortable. Making a good decision to buy a puppy means putting aside emotions and looking at facts. Someone uninvolved can be a great help in doing just that.
I have seen many people who are otherwise very rational become very irrational when the holding a cute, loveable little fur-ball. You will be living with this puppy for more than a decade if things work out well. Saying NO to a few puppies before you say YES to one is a great idea.
The opposite of taking someone emotionally uninvolved with you is taking kids with you. The puppy could have three legs (all in the front) and the head installed backward and the kids would still fall in immediate and permanent love with it. Show the kids the puppy AFTER it passes your test. If possible, show it to them AFTER it passes the Vet Check. Why get them emotionally attached only to find that it has a heart condition, or a kennel-disease? (You should avoid getting emotionally attached, too.) At this point, it is just a candidate. There are hundreds more out there just like it and more are being made everyday.
I know it sounds cold, but buying a puppy has to be an unemotional business decision. Once the deal is done and the puppy passes the vet check, then you are REQUIRED to fall head-over-heals in love with it, but not a minute before.