I think what makes a farm, is that the trained players are way too good for the league (they're NT players), but the team is still not trying to win games in the competition. The sole purpose is to devote all resources of the team to make a couple super players for the country's NT, not to help the team win games at a high level.
If a team has 2-3 NT players (of a decent sized country with a decent WR), it should probably be a team that's at or near the top of the country's competition (depending on the size of the country). If the team is not near the top, it's probably a farm because it has the resources to compete but doesn't. It's not logical for a manager with 1 team to not compete at that point, so the logical explanation would be that it's a farm team of a manager that has (an)other team(s) that do compete.
Note that this description excludes rebuilding teams with NT players, because those teams do compete at some point (even if it's after a long period of devoting everything to training and not competing). A lame manager won't be able to train NT players, so those are excluded as well.