SRTP usually, ZRTP less often, and VOIP over VPN least often. As long as both sides of the connection support SRTP or ZRTP and are configured to kick it in as needed, usually all I do is check for the lock icon.
Any clients which support OTR should be encrypted easily. Adium is pretty standard on OS X; Pidgin or Miranda work fine on the Windows end. And if the other person doesn't support OTR, the system falls back to unencrypted.
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