Tell if an OS feature exists?

FROM : Mike O'Connor
DATE : Mon Apr 11 05:45:33 2005

My Cocoa app works fine in the latest system, but it uses a feature
that is not in earlier systems. How can I tell at runtime if it is
there or not?

Should I check the system version? This seems crude.

I use NS... classes that are not in older systems. Can I test to see if
the NSSomething (AppKit) class exists, if so how?

I use stuff in a system framework that isn't there in older versions.
How do I test to see if it exists?

TIA!