I have a new problem. With DDG is search to limit torrent downloads and found Smart Queue Management which I installed.
I nothing understand.
I wish ONLY to limit torrents, because they eat my normal surfing bandwith. Is this Smart Queue Management the right way? I understand not in this configuration.
In that case try sqm with per-internal-IP-fairness (especially the Making cake sing and dance, on a tight rope without a safety net (aka advanced features) section is of relevance). This will essentially distribute bandwidth equitably between active machines, and a torrenting machine will get approximately its "fair" share as will everybody else. That is not exactly what you asked for, but should effectively ameliorate the issue, maybe enough to disappear from your radar...
From the outside you really can not robustly and reliably, simply based on behavior. Deep packet inspection might all you to do this, assuming the torrents are not using encrypted protocols/tunnels, then you would need to break the encryption and/or man-in-the-middle it to snoop the torrent-ness of the traffic.
IMHO just giving each end-host a fair share of the available bandwidth is a much simpler heuristic that works reasonably well...