It takes more time and effort, but seeing your potential future bandmates in action is the best way. It means that you may have to go to the venues that these folks play in, and I don't know if you're old enough to get into some of these places. If you are old enough, try not to get sucked in by the drink-your-face-off-at-the-open-mic scenario.
You can see how these people handle themselves in a real world situation. You may get a chance to sit in. Keep in mind that the craig's list and music paper ads will never show anyone in their REAL light. You could get more by seeing someone in action. How they play, how the interact with others, etc.
You can stay anonymous this way, with no commitment on your part. If the person or persons aren't to your liking, then you can leave. no harm, no foul.
If you're not of bar age, visit the local schools, and put the word out in the music departments and student activity boards. Go to local events where music is featured.
If you have an instrumental teacher, that's a good first source. Local music schools. Local music stores.
It's one of those things where you may have to kiss a few frogs to find the right person.
And, as with any situations where you're meeting someone for the first time, keep your wits about you, and be careful......