How to tell a native speaker

I just learned how to tell a native speaker of English from someone who has mastered English as a second language. 

In English we don't pronounce words the way they are spelled. And some words (in some contexts) get a weak pronunciation. 

As a native speaker, I might say: "I'm driving to Healdsburg." Unless I seek a special emphasis, I will pronounce the "to" as a barely audible "tuh." A second language speaker will usually give "to" a full-on treatment. 

I guess we give about 25% of our words the weak treatment. We absorb how to do this as young children, and we don't give it any thought.