How can I tell when red bananas are ripe?
They do change color, just not so obviously as normal bananas. There's a slight greenish tint that disappears as they ripen, and the red becomes a bit deeper (some people say purplish).

They also get softer - including the skin getting more tender, just like yellow banana skin. Just think about how bananas feel when ripe, feel your red bananas, and (perhaps with a tiny bit of trial and error) you can figure it out.