A Scumtell is a particular behavior which a player believes is indicative of someone being scum. For instance, lying is considered by many to be a very strong scumtell, under the belief that scum always lie and town should never lie, thus catching someone in a lie is a good sign that they're scum.
Scumtells are neither universal nor absolute; they can and do vary wildly from player to player. Some players rely exclusively on their list of scumtells, while others deny the existence of scumtells entirely.
See also: WIFOM.