When it comes to testing a multi-threaded app, there is no substitute for getting a few hours on a multiple-processor system. All your tests which you ran on the single-processor machine are valid, but only a part of the testing you'll need to do. You'll soon discover the difference between empirical evidence and proven robustness.

What's worse is that each new threading model can also affect borderline code. What worked on POSIX will have a race condition on green threads, or vice versa. Knowing your OS's threading model won't help very much from an application level: you just have to get it right, not merely apparently okay. It'll come from experience.

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