Pronunciation of a spell is just a tool to help young wizards and witches learn how to control their powers. Spells do not require speaking, and most of the powerful magic is down without speaking. And most spells require emotional or other components as well, not just the words. The way you move the wand. What you are thinking. Your conviction. In the fourth book, Professor Moody tells the students in his Defense Against the Dark Arts class that:
Avada Kedavra's a curse that needs a powerful bit of magic behind it — you could all get your wands out and point them at me and say the words, and I doubt I'd get so much as a nosebleed.
Later when Bellatrix kills Siruis, Voldemort taunts Harry.
In the books, Bellatrix tells Harry he has to want the Cruciatis curse to inflict pain for it to work well.
That said, Magic in Harry Potter is a combination of natural talent, practice, and raw magical capability, just like any other skill in real life. Just cause you can speak doesn't mean you speak well. Just cause you can run doesn't mean you will succeed in a marathon or race.
As to how a stranger can tell the difference between a weak and strong wizard, it's really up to being shown the difference. Aside from the typical correlation of age to power, you can't tell a wizard's talents just by looking at them. (Young wizards tend to be less powerful than adult wizards.)
The most powerful wizards are able to do wandless magic.