By doing research.
The only way to truly know which model to get is by knowing what you want from the given product and perhaps more importantly, how much you are willing to spend.
Car manufacturers for instance market their cars by telling you how much the car's price starts from, while showing you the most expensive and the model with the highest specification.
Some of the add-ons they provide may be bigger engine, alloy wheels, sat-nav, leather upholstery, fancy paint jobs etc. Each of these options will add cost to the base price and basically it becomes a question of "how much can you afford"?
Where engine size is concerned - certainly in the European market - the cars with the smaller engines tend to be limited often severely. You can end up with a car that looks good, but has no get up and go, struggles on hills or with more than one person inside etc.
With other items where differing models are concerned, your requirement should be the foremost question in your mind.
If you are looking to purchase tools or musical instruments, hi-fi or TV, where the quality of what you're buying can mean a great deal, then try and buy the best you can.
With any of the above, you get what you pay for, but it's up to you to read specifications, reviews and so forth so that you can differentiate between good quality products and products that are expensive because of frivolous 'gadgets' attached that make little or no difference to the performance.
People's reviews can help you make your mind up too as they have tried and tested the product and can give an opinion - sometimes very much more honest that the manufacturers - about the performance, longevity, robustness, usability and value for money of a given product.
In short, you need to do your homework.