Hi eagle737,
There are lots of suggestions there and most point to having at least two sources indicate a climb.
In the simulator with an engine failure on take off, I've seen some excited crews call "+ve climb" when we are about 5 feet radio, and the aircraft is hardly climbing yet. The open gear doors create more drag initially, and some crews may fly level for a while or even sink a little.
So the "+ve climb" call needs to be made after being certain the aircraft is safely climbing away from the runway, and not just an automatic call having seen one instrument twitch. The day you have a real engine failure on take off is the one time you don't want to make that call too early.