When I was in the air wing in the '70s, I had the unpleasant opportunity of pulling wheel watch. The unfortunate individual sat in a small shack (maybe 3 x 6 ft.) with a pair of high-powered binoculars ensuring that each and every aircraft on final had their gears down. If not, there were two flare guns pointed at opposite right angles to the runway that had to be fired to warn the pilot off. I know I had to use them on more than one occasion. I think it was a thirty-day hitch. It seems like such a primitive way to ensure the gears were down, and I wonder if it's still being used today?
They do not continue this practice anymore. This is likely because landing gear are more reliable now than they were in the 70's. There is also the practice of Air Traffic Control prompting military pilots to check their gear is down, by verbally adding "Check Gear Down" to their landing clearance.
Source: Checked in with some of the Air Force / Air National Guard pilots in the office. They're not aware of any such practice and decided it must have been before their time. Oldest pilot started in the 90's, so that leaves you with a 20 year gap where they abolished the practice.
In the early 1990s the Navy used to deploy instructor and student pilots to perform wheels watch at outlying fields that were NOT tower controlled, and where student solo pilots would practice landings during primary flight training in the T-34C. Without an instructor or tower this practice made sense. I cannot speak to whether or not it is on-going.
Once complete with primary training, any landing practice at non-tower controlled outlying fields would be done with a carrier Landing Signals Officer on station. The LSO would note trends, grade all landings and also function as a wheels watch, although wheels watch wasn't the primary purpose.
During 20 years spanning the mid 1980s to early 2000s I was never aware of any wheels watch at a tower controlled military airfield, but that's not to say it didn't happen somewhere.
I remember the AF still had the runway supervisory unit (RSU) when I was still on active duty in the '89-'92 time frame. I wasn't an AF pilot, but I was a private pilot flying with the Aero Club at the time. I'm pretty sure the RSU wasn't staffed full time. It may have been limited to periods of high op tempo when the consequences of a gear up landing would be more severe. I don't remember seeing an RSU in the years since then.
Edit: I was reminded that the field got a new control tower shortly after I left. I believe the RSU was removed at that time as it was no longer needed. The new tower provided much better visibility to that runway end than the old tower.
We did have the 'check gear down' calls from the tower in those days. I think the RSU became redundant to the ATC call/Pilot confirmation. For a/c with 2 crew, you had a backup check on board. Most of the fighters by that time had the landing lights on the gear, so in addition to the ATC call, the tower could just look for the landing lights.
As for the reason for the calls, it's not just about the potential loss of the a/c. In a combat environment, the runway is a critical asset. A gear up landing would render the runway unusable for a time preventing other a/c from landing or taking off. That can seriously impact the mission. Similarly, the Navy has been known to push an a/c overboard to clear the deck of a carrier.
The second factor is the culture. In the civil world ATC has their job and pilots have theirs. They don't want to cross that boundary. In the military, everyone is part of the team and they work together to help ensure mission success.
As a current student naval aviator, I can tell you that Wheels Watch (who is a student assisting the Runway Duty Officer) is something that is still alive and well for the Navy Outlying Fields that we train at in addition to home field (North Whiting KNSE), which has a controlling tower.
To speak to the operational necessity of this safety measure, T-6B's still manage to have gear up landings at OLFs, so when you have an excess of warm bodies to throw at a problem you are inclined to keep using them.