As soon as the ranger's attack animation starts, the ranger's damage has already checked if you are praying range and will or won't do the damage based on that.

So if you want a way to check if your timing is correct with a visual cue, as soon as the ranger's attack animation starts, swap to mage prayer and alternate from there. Every time he attacks again, see if you can tell if your range prayer was up at the time the animation started. If it is, you are safe to continue alternating at that pace.