The best is to have a couple of different smartphones and tablets to try on. If you're trying to optimize your game, it's good to have some, lower/mid end phone to try on. If you're developing for iPhone i suggest you get an iPhone. Maybe a second hand iphone 5 is good, and for android some cheaper mid range phone. You don't wanna test on the latest most powerful phones only, because you need to know how well your game runs on phones that are not that powerful.
I do my testing by uploading the project to my server, then trying the game in the phone browser. You can also set up Construct to preview on LAN, so you can test directly on your phone/tablet without uploading anywhere.
The phone browser performance will usually be very close to final build performance.