AskHistorians

They had some idea, although it was less certain than seen in the context of Saving Private Ryan.

First of all, there was efforts to weaken the defenses. Both bombardment from naval ships and dropped bombs from planes were meant to significantly soften the defenses for the landing. According to the plans, this should have significantly reduced the defenses. In reality, the naval bombardment was nowhere near large enough, and the bombers missed their targets due to bad weather. This was only discovered as they reached the beach.

Once the infantry was landing, there was also supposed to be quite a bit more support for them. Specialized amphibious tanks were created, and meant to be driven up onto the beaches to provide cover for the infantry. This almost immediately went ary as the rough water swamped or sunk dozens of the initial tanks, and lead to the use of landing craft for the remaining tanks, slowing down their deployment. Even of those that were launched by landing craft, many were lost.

Also worth noting is that the Normandy beaches, don't actually look that much like the movie. When they land in Saving Private Ryan, it looks like they're only 20 meters or so from the cliff. In actuality, it was much, much further. If you look at the famous photo from the landing, Into the Jaws of Death by Robert F. Sargent, if gives you an idea of what the beach actually looks like and the conditions on the morning - visibility was low, and they were likely a hundred meters or more from the cliff face. Less likely to get shot the moment the gates open then how it looks in the movie, although horrifically even worse in nearly every other way.

If you're really interested in more detail, TimeGhost has an excellent documentary (split into pieces to make it watchable as a series) on the subject on their D-Day 24hrs channel that covers the background, the events of the day, and the details and context surrounding it in extreme detail. That said, its a multi-day watch, given that its 24 hours long.