Residency is conditional the first 2 years of marriage. Both spouses need to go through interviews, appear they are a couple - have joint bank accounts, bills, live together, etc.
It's also not a given that somebody with no existing legal status will be granted residency even if legitimately married.
There is a lot of variability in what the USCIS actually checks. When my wife and I went for our interview the immigration officer only asked to see our wedding photos; since our parents and other family were present he decided it was legitimate (apparently most sham marriages are hidden from families). We had spent a lot of time assembling all the other recommended evidence but no one ever bothered to look at it.
