One way, I found to see if rabbits are overweight is to stand behind them and put something interesting, perhaps a treat on top of something not too high, but still quite big, in front of the bunny. The rabbit will be curious and will warily inspect the object by inching forward. When they do this, if you can see their belly between their back legs, they're getting a little fat. LOL, I don't know how I found that out

A rabbit leash is a brilliant idea, although not all rabbits will wear them. My first rabbit, Bo-peep, absolutely loved it, but my second bunny Cookie, wore it whilst he was a baby, then suddenly turned against it and got really frustrated when you tried to put it on. It took a long time for me to get him accustommed to it again, but it is possible. I still have his bunny lead, I would send it to you but it's very old and tatty now as it's been in the shed for a while now and there are still marks and rips where Cookie bit it.