I ask this because I've not once had any indication of a critical hit with my weapons, outside of V.A.T.S. Are they no longer a random factor in combat like in New Vegas?
No - critical hits are now tied directly into VATS and do not occur as a randomly chance when attacking an enemy. As discussed here by game director Todd Howard:
Another change being made to VATS is the way that critical hits work in the game. Previously, critical hits, marked by more explosive and devastating shots, were random. Now players will be able to choose which one of their shots in VATS will be the critical hit. This will allow players to be a bit more creative during combat, and should result in a more engaging combat experience overall.
Note that a critical hit is different from a sneak attack.
Yes, critical hits can be made outside of V.A.T.S. / VATS, but only if you take the drug "Overdrive" beforehand. The following link doesn't mention it, but I have noticed it in-game, that if you are under the influence of Overdrive while firing from the hip, you will score Criticals every few shots. You'll see the text "Critical Strike on so-and-so" in the upper-left corner of the screen, but it will look like a normal shot (no killcam). I had no mods installed when testing this.
+25% Critical Chance for 8 minutes
For your information and not related to the question, Overdrive and Psycho stack, so if you want to get a free +50% damage, take both of those at the same time. (Psycho also gives +25% Damage)
Critical attacks can only be performed in V.A T.S, while sneak attacks can only be performed while sneaking.
While critical hits are 'built up', and perform devastating damage, a sneak attack is somewhat different.
If you are sneaking and the NPC you hit has not detected you, you will perform a sneak attack. These hits are simply akin to regular hits, with increased damage.
At the base level, a sneak attack will inflict 2x damage, but this multiplier can be raised.