You just have to be skilled to be effective in Fallout.
Also, pay attention to your clothing and how fast your moving, you are more easily spotted when running, the pip boy light is on, your wearing heavy armor etc. Also, your weapon has a lot to do with this, if you don't want to be discovered after you take your first shot, get a silencer.
Stealth Boys are key in some areas until your skills are up to the task.
Even then, you can be sneaking under the best of conditions and someone will still smell you or hear you (Deathclaws anyone?)