The best way to choose an electric bike is to test-ride different kinds of electric bikes. If you have a change to test-ride different electric bikes, this is by far the best way to make your final choice.
Don't think for a moment about all the reviews that you've read and all the other technical stuff that you came across. Just enjoy the e-ride at first and see if you like it. It's not until you step on an e-bike that you really know how it feels like and how much fun it is.
First and foremost, you should feel confident and comfortable, don't forget to set the seat height properly and adjust the handlebar and grips position. Try different bikes and compare the dry data of one e-bike to those of another and see if it holds water.
The handling, especially in turns, should be smooth. In some e-bikes, the electric components ruins the delicate bike balance. You would feel like you are not fully in control. Try riding with no hands. With a good electric bike, it should be easy to maintain a straight line at low and high speeds, there should be no wobble of the front wheel.
Placing the motor at the rear or front wheel is popular but it increases the weight of the wheels significantly (unsprung weight), therefore causing the bike to poorly handle bumps and road vibration. It degrades the ability of controlling the wheel under hard acceleration or braking. If you are considering buying a mountain bicycle for rough off-road cycling or even if your cycling path is bumpy, it can ruin your riding experience.
Check that all the electric parts are well protected and sealed against rain and dirt, and if you are given permission go sloshing through off-road muddy creeks to check the e-bike liability.
Slide the battery inside, out from it's place. It should be easy and comfortable. If you need to struggle in order to get the battery in or out, it is not a good sign. An easy struggle now can turn into a big struggle of even a failure in the future.
Check the console of the e-bike. Get familiarize with it and decide if it is good enough for you. Check the e-bikes console page for more details.
Check the lights (if there are any). If it possible, ride the e-bike at night and see if the lights are good enough. Otherwise, compromise on checking the lights in some dark area.
Do a lot of gears shifting. Shift from the lowest to the highest gear and vice versa. The shifting should be smooth and quiet, the motor should not restrict the shifting. In some poorly designed electric bikes it is. It can makes the chain fall from it's place or even cause it to break apart.
Electric bicycles have an higher average speed then conventional bicycles so the braking quality is more then important. Also, the brake levers should be positioned in an easy to reach position. Beside mechanically braking the bicycle, it is preferable that the brakes will also cease out the motor operation. In some countries, it is statutory.
If the law in your country permits using both pedelecs and power-on-demand e-bike, try both and decide what is your personal preference.
If you have the e-bike for more then a couple of minutes (preferably a number of days), take it for a long ride and check the range. First and foremost, does it offers enough range for your everyday errands. Second, observe how the range is affected by different types of terrain and different pedaling power.
Ride the e-bike without the motor assistance. See if it is as easy to ride as a regular bicycle or not. Pay particular attention to the added motor friction. Does that friction unnoticeable, bearable or too much to handle?
If you are test riding a bicycle with a conversion kit, ask to ride on the same bicycle without the kit in order to feel the difference. Pay attention to the added weight, balance and etc.
If you are considering a folding electric bicycle, fold the e-bike, see if it fold nicely. Lift it and see if it is light enough for you and if it is comfortable to carry on. Also, check whether the electric parts are messing with the folding mechanisms. Do they get stuck in the middle and therefore the folding is not complete?
If one of the top reasons for considering an e-bike is some sort of disability, concentrate of how the e-bike feels in relation the your specific disability.
Finally, one of the top reasons to get an e-bike is to make riding hilly terrain a snap. If you live in a hilly region, ride the e-bike up and down - look for how it behaves at low gears while climbing, how the motor behaves at different gears and is it really giving you a nice boost? When going down, check the handling, balancing and brakes again. You can also try to go up and down without the electric assistance and see if the added weight and friction are not too much of a struggle for you.
With so many variables, the only real way to tell if an electric bike is right for you is to take it for a test ride, preferably on the terrain you most likely to ride on.