The Shift+Tab scoreboard can give you an idea of which team has the most attack, since it shows the highest attackers first. But it lists players that are not sunk yet above players that have, so you either have to do some guess work or wait until only 7 players remain on the enemy team (so you can see the highest attack of all sunken players too). Players that have left the battle are displayed even lower I believe.

music is different for winning and losing while in game

ask for team attack in all-chat

To me, it's all according to guesswork. The music thing works SOMETIMES ( I don't use it to judge whether the team is winning/losing), so I usually refer to the player score, and watch how the BB/CV plays. If the CVs aren't giving enough sight, exit. If the CVs are doing a decent job, but the BBs look like they are all over the place, exit.
The only tell tale sign of whether your team will win in the end to me really, usually depends more on the skill of the players, rather than the size of the team. (To a degree of course) 

Only issue with the music is that sometimes it kicks in too late, and it doesnt adaptively change if your team starts to turn a loss into a win (I don't think it does anyway, I haven't actually had the music on for years).