Scour online comparisons and reviews, such as those found at the "Popular Mechanics" and "Consumer Reports" websites by entering their titles plus "Backpack Leaf Blowers" into separate search engine searches. If you are interested in a commercial-duty blower, add that to your search wording.
From the results of your online search, make a list of the blower models you are interested in shopping for and their particular features like engine size, air power, rating for noise level (although, you should always wear ear protection when operating a blower), weight and price.
Venture out shopping and begin by trying the different models on for size, comfort, well-placed handle, visibility of gas level, fuel spout accessibility, and ease of starting and use. Remember that the backpack leaf blower will be heavier when it is full of gas, so comfort is important.
As you may not be able to start a gas backpack blower in the store, ask about the stores return and refund policies and keep your receipt after purchasing.