There’s three very good reasons why we know it’s not faked.
1) It would’ve been harder to fake to the degree they did than to just go there. How many other movie special effects from 1969 stand up today?
2) We left stuff behind! We can see it, today. There’s even a big mirror that we use to bounce lasers off.
3) The most compelling of all, if it were faked, the Russians would have gone “er, excuse me…”
Do you really think, in the great Space Race, the US would have got away with saying to the soviets, “well, we did it. No, really, look, we filmed it and everything. No no, don’t bother looking up, trust us.”
Most – nay, all – of the conspiracy theories are easily debunked, and are borne out of the same intellectually challenged logic that gave us Creationism; to wit, “I don’t understand it, so it can’t be true.”