I don't know about other airplanes, but there is plenty of feedback in the brake pedals on the 744, such that I can discern when they are releasing for a slippery runway. That holds true when using Autobrakes as well, as long as I rest my feet on the pedals.
Manual braking practice is definitely a good idea! Get a good idea of speed vs runway distance marker on a familiar runway when using your normal autobrake setting. Attempt to replicate that using manual braking. Just don't use heavy braking at first - let reverse thrust do its work, and slowly add braking force.