How is this even being debated? Aluminum is very light, very soft. If your watch is heavy, as expected, it is steel. If it is light like a plastic watch, it's aluminum. If you can tell the difference in weight between a titanium watch and a steel watch, you can tell the difference between aluminum and steel. If you still aren't sure, you can take an Xacto knife and try to cut a chunk out of the watch in a hidden spot. If you can cut a chunk out it's aluminum, if you just end up scratching your watch but can't get chunks/shavings, it's steel.
I doubt that any watch maker would use aluminum.