Personally, I like to use the Mark I eyeball. If the houses are getting smaller, we're climbing. I use to work for a little known airline, TWA, and the only call was "Gear Up". The pilot flying had to be aware. Of course, like everything, "trust but verify". BTW, this technique has worked for my 37 years of aviation. Another way to look at it, is if you have rotated, you are past V1 and then some with no other way to go but up. But for all the lawyers, it sounds good on tape.
fwiw,
gumby