How do I know my wallet.dat actually already contains 100+ addresses ready to be used? I'm using the standard commandline client (under OSX, in case this matters).

I've read in some threads that it is smart enough to pregenerate 100+ addresses to be used in future when you ask them, so that if you backup an earlier wallet, the future private keys are not lost.

This would be cool, but how do I check it is really true?

Every time I "get a new address", it seemed to me it took enough time to suspect it was generating it on the fly, not using it from a pregenerated pool. Maybe it was slow for other reasons, though, and that's why I'd like to check somehow.
you can check by directly exploring the content of your wallet.dat file, which is a bdb database. there are various tools for exploring the wallet, one of which is gavin's bitcointools: https://github.com/gavinandresen/bitcointools

Another exporter is pywallet.py.