Scum are distinguished from town by exactly three attributes:
1) Objective
2) Information
3) Number
I misspoke when I said there were two.
The objective of town is to successfully identify the alignment of each player in the game, and secure the lynch of those who are scum. Since they begin the game with no knowledge of the roles, they find themselves in an environment extremely hostile to these ends. Thus, town are in a position where in order to win they have to generate as much information as possible about each player, before forcefully arguing for the lynches they can never be %100 confident in. In order to get these lynches through, they have to cooperate with players they have few reasons to trust, but are generally more likely to be town than otherwise. Also, the town outnumber the mafia by quite a bit, meaning the cost of an individual town person dying will in most cases be much less than the cost of an individual mafioso dying.
The objective of scum is to prevent the town from successfully identifying them, and redirect the lynch to people who are actually town. Since they begin the game fully cognizant of the roles (at least with respect to town-aligned versus non-town aligned), scum can only win by making arguments they know to be false. Thus, in order to win scum have to prevent as much information from being generated as possible, as they agree with and argue in favor of reasoning that in the back of their heads they know to be wrong. They also do not have much incentive to go above and beyond, because everything they post risks a misstep, and all they really have to do is avoid the lynch. At first, with almost zero information and a large number of townies to mislynch, this is not a terrifying prospect. However, the only way they can avoid the lynch for long is by successfully
simulating
the objectives, access to information, and confidence in numbers of the town.
A couple things we can note here that follow directly from these differences:
- Town will tend to want more information rather than less, more activity rather than less, and more transparency from the players who are not them rather than less. After all, they don't know the answer to the puzzle, and victory depends on figuring it out.
- Town will tend to be insecure in their reads, because in most cases they cannot ever actually know for certain that they have chosen correctly. This insecurity will
not
necessarily manifest itself in nervousness or indecisiveness, as many town players (experienced and inexperienced alike) present their reads confidently as a tactic or as part of their personality. However, in most cases a basic uncertainty will show up elsewhere - they'll keep combing the thread, they'll keep asking questions, they'll keep up the pressure on whoever they've voted for in the hope of information that will confirm their read of the game. They'll never get this confirmation, and the paranoia will never truly leave.
- Town will tend to be less concerned than mafia when suspicion is directed at them. This is because 1) they
know
that they are town, and 2) dying is not as catastrophic to their win condition as it is for a mafioso (PRs complicate this a little). As a result of this, town tends to be bolder, more determined, and less taken aback by minor things.
- Scum will tend to contradict itself. This is because they do not actually have opinions. Only goals. Since their beliefs are simulated, they require effort to maintain.
- Scum will tend to over-explain. In the back of their head they know their reasoning is fabricated. Fear of discovery leads them to paste things over with excess contingencies and justifications.
- Scum will tend to hide behind logic. Scum
love
logic because it works the same way for them as it does for town. It's easy to come up with an airtight rationale for doing just about anything. It's
not
easy to fake a fluid and impulsive gut read. That requires real skill, and scum avoid doing it when they can.
- Scum will tend to lurk. The less they post, the less they have to fabricate, the less they slip. Also, they don't actually care about solving the puzzle, so when they feel confident about things politically, they don't have the same incentive to show up after hours.
- Scum will tend to tunnel. It's easy. Also, if an ineffective player provides a plausible enough reason for being suspected, scum can minimize their risk by leaving their vote parked in one place and justifying it repeatedly instead of making more unstable plays.
Of course, none of these tells will work universally, and when they do make their appearance, it's not always in the way you would expect. For example, it's a common scum tactic to feign being totally confident by going to the opposite extreme (i.e. brushing off attacks sarcastically). It takes a smart town player to see through this smoke-screen and identify it as a another type of nervousness. Similar things apply to everything I've said above.
This is worth taking a step further. You can usefully think of everything I've written above as the "Tier 1" of mafia psychology. It's the obvious, essential core of mafia; the ultimate cause of all differences in behavior between town and scum. However, in order to get to "Tier 2" you have to take into account the fact that the scum is aware of Tier 1 and is actively trying to avoid making those tells. For example, you call this a Tier 2 tell: "scum knows that town is suspicious of tunneling, so they tend to go out of their way to post about
everyone
". Then Tier 3 is when the scum has taken steps to account for Tier 2, and you're off! You're now spiraling indefinitely up the WIFOM ladder, never to return.
The short answer: What good scum-hunters are looking for more than any one specific thing is insincerity. Exaggerations, distortions, manipulations. You're trying to find calculated, self-conscious, over-confident, or lazy play that is not easily explainable by the poster's apparent personality. You have to form a model of the person you are reading based on their personality, and then contrast that model with what they are doing. Then you have to trust your gut. It's really that simple.