No expert, but rabbits dropping dead en masse is imho the main symptom. So, if you have wild rabbits around, and suddenly there are very few of them or even dead ones - high alert, time to act paranoid about the outsides.
Of course, vaccinating is a good idea, even if you maintain really good quarantine proceedures, vaccinating sure is better.
Back when I was new to rabbits I had no idea that something like Myxo existed, so I didn't think much about that wild rabbit I found on the paintball field, still alive but in awful condition, some weeks later 14 of my 15 rabbits died. In retrospect, I had noticed that the wild rabbits had disappeared.