Half-orcs from what I recall can vary quite a bit in terms of appearance. IIRC in 3.X there was mention of orc tribes sometimes using half orcs to pose as half-elves for espionage purposes. I think Ed Greenwood is on record saying that in some of the orc/human barbarian tribes of the north there really isn't much of a distinction between orcs and humans and they have children together readily. Probably why Uthgardt etc barbarians all look like Arnold Schwarzenegger.
Most of them look distinctly orc-like though, and not just a human with little pointy teeth, going by the art.
In AD&D 1e, "10% of Half-Orcs can pass themselves off as the other half." I don't know if later editions revisited this "racial" trait. Still, these half breeds are taking a hit to Charisma; even if they do look human, it's an unattractive Human (most of the time).
