F-111 emergency landing with no wheel; what is done to reduce risks when loosing a wheel?
What are the relative risks of an ejection vs a no-wheels belly landing. I mean obviously the costly loss of airframe. But is the risk to life pretty even odds?
  
  In a conventional aircraft (e.g. civillian) ejecting is just not an option so belly landings are the norm. But in an F-111 is a belly landing still safer than a controlled, planned ejection?



Ejecting is an extremely violent event, often resulting in the pilot being removed from service for an extended time period, or permanently. 1 in 3 suffer back fractures or other major complications. Ejecting is possibly an end-of-career move for pilots (at best, an end-of-life one at worst). Belly landing on the other hand isn't usually a bad event, typically the aircraft is repaired and returned to service. In an ejection, the aircraft is a total loss, not to mention a rather large unguided bomb.



  Are there no sensors etc. that'd detect they had lost a wheel? Any other indirect indications based on asymmetric drag, hydraulics etc. that one may expect? In other words, how likely is it that one loses a whole wheel but does not know.



No, there are no "wheel fell off" sensors. There are possible sensors to tell the pilot of low tire pressure, and certainly have sensors for hydraulic pressure. That isn't to say though that getting a low tire pressure means a wheels-up landing, landing on a flat tire is entirely possible. Drag probably isn't going to be significant, at least not enough to notice outside of gusts or other factors. That being said, having a wheel "fall off" is an extremely rare event, probably so few that you could count them by hand. 



  Are there standard procedures where ATC uses binoculars etc. to check visually whether all seems OK with a landing aircraft (wheels etc.) I know they will check on request, but is there any standard protocol?



Not typically, but ATC often watches departing aircraft with binoculars, not necessarily looking for missing parts though. In clear visual conditions, it would probably be pretty apparent that a wheel fell off to people on the ground, at the very least the next departing aircraft who had to avoid it on the runway. Usually when something falls off the aircraft, the runway needs to be closed while they do a FOD (foreign object debris) removal run. 



  Don't military airfields lay down foam for emergency landings? Or is there a downside to foam on the runway?



Yes, military airports can lay down AFFF (aqueous film-forming foam) on the runway, but you don't always need to. The down side is that it causes the aircraft to slide quite a bit further than it may be planned, and it's incredibly hard to clean out. If there isn't much of a risk of fire, its better to just belly it down in the grass. Or just let it slide on the runway. Most military aircraft can dump fuel and land without much fire risk.




By the way, if you think it is impressive to belly-land an F-111 after a wheel falls off, you should watch this video about an Israeli pilot who landed an F-15 with one wing!!!.