how to detect appletv for mirroring?

Be on same network, apple id with homesharing on for ATV and Mac or iOS. Always try a reboot of router and ATV with all cables out for 30 Seconds too. Mirror icon should appear near clock IF you have a mid 2011 mac or newer (early 2011 for MBP) AND are running OSX 10.8 Mountain Lion or newer.

Oler mac or OS can 'mirror' with a third party app called airparrot.