For me its a mix of some youtube channels and BoardGameGeek. On the YouTube side there is Tabletop which can be fun. I've seen games on it that made skip the game because I didn't find it interesting or the game mechanics weren't what I wanted in a game. And while some people may not like them some of the reviews people put on Amazon can be informative.
Finally my favorite way to discover a new game to play is when I go to PAX East. I'd sit down and learn a game on the fly. If its easy enough to learn I may buy it that weekend and enjoy with my friends during late nights or early mornings of our trip. The game Ascension is a good example of it. We learned it at their booth. Picked up a copy and played it at the hotel later on at night.
There was a board game thread not long ago. I tried to find it but had no success. People listed a bunch of games they liked. These are the ones we play the most at my place:
Ticket to Ride - Very easy to learn and fun.
Settlers of Cattan
Cyclades - Fun game. Reminds me of Clash of the Titans a bit.
Ascension - Easy to learn. It has many different editions out, so I suggest going with the newest one.
Cards Against Humanity
There is a Marvel cooperative card game that is a bit long to set up but fun. I think there is an Alien version of it out now. Resident Evil was another card based game we've had a good time with. I played Star Realms at Pax this year and had a good time with it. Its a two player space ship combat card game. A game that was harder to learn (for me anyway), but good once you get it going was Puerto Rico. Another one I'm a bit slow to get is Android: Net Runner.
I could be misremembering but I think I've heard Pandemic is good game.
*fixed some misspelled words
Pandemic is a great game, full of cooperative strategy and varying levels of difficulty.
Is the Marvel cooperative card game you're thinking of Sentinels of the Multiverse? Or is it something else?