Don't see any way that fans can know whether a captain is any good or whether someone else would make a good captain. It's very much an internal thing I think; which players are respected and listened to, which players are able to keep calm, who can influence people whether quietly or aggressively. Fans will basically say anyone who captains the team to a trophy is great and anyone who doesn't is useless. See everyone saying Coleman must be the next captain because he was so clearly made for the job, and now saying he's useless.