Early on during a Scout's career, Stealth will be detected by any individual that considers as red to them - about a four or five levels difference. In the mid-10s levels, however, this level difference starts widening - for instance a level 17 Scout will be able to Stealth past most level 25 opponents.
Some types of opponents can always see through Stealth, as a special ability. This includes many named opponents, and various monster types designated as 'lookouts', 'sentries', 'scouts', etc. The name of those opponents will be highlighted in red, and careful Scouts can thus mouse over opponents to see if they can detect them.