Stealth vs. Observation/Perception

[Marc]

Bona tarda!!

This is about an old story that everybody tends to solve in a different way. Strangely I've not found a satisfactory way to resolve it in any RPG system without the use of House Rules. So, after running the last session I'm curious to know How do you manage the Stealth vs. Observation (Perception would be a more suitable term) procedure? What factors do you take into account to modify the chances of the observer and the character trying to avoid being detected? How do you translate these factors to a modifier for the dice roll? Do you simply solve it as a normal opposite tasks without modifiers, each character rolling vs. the proper asset and checking who has achieved the success with a greater margin?

Thanks in advance.