An Alt is just a tool but Cata's right you cannot detect a silent Alt.
Also level 25-40 can be reached quite quickly if your playing to get a high level.
Another way to tell if someone's an Alt is if they are much very skilled and low Level or haven't played many 1v1 games but are very good on less used templates.
Most people don't use alts to troll, they use them to play Europe 3v3 in most cases. In a few occasions have people cheated using alts, but it is rarely a problem in general.
I will create an alt between now and 5 weeks, the first person to catch it gets a shout out on my profile, because that's about the best thing I can give.
2v2 ladder template isn't the most strategic, nor the most well played. RoR 3v3 / 4v4 I think is more often played, but I'd agree that Europe 3v3 is the most strategic.
As a whole, I see more Rise of Rome games than Europe games. However, these games tend to be played by less skilled players than Europe games, and I see 1v1, 2v2, 3v3, and 4v4 all on Rise of Rome Full Distribution. For Europe it just tends to be 2v2 Szeuropa, and 3v3 Europe (ofc with the different variations).
I could very well be wrong, but I think I see more strong players play on the Europe map as opposed to RoR, for team games. I seem to normally see more games on RoR map though, in open games.