Rank tells you nothing. MMR doesn't. Win rate doesn't either. In order to get any idea what you're up against, you need to go further than that. What I do is check out my lane opponent's stats with their chosen champion. Like last night, I played my first ranked game in over 2 weeks, and was facing a Tryndamere. When I looked at my in-game info, it said he had a KDA of 6 with the guy. Upon further inspection, however, I saw he'd played a total of **1 ** ranked game with him, telling me he probably got lucky and that I likely had the edge (119 with Garen, 5.07 KDA, 53%). Early on, I probed him for weakness to confirm my suspicion, found I could zone him out of CS with ease as he ran or flashed at the first sign of danger, and bullied him with confidence the entire match, doubling his CS and finishing 3/0/5.
That same match, I mentioned that I hadn't played ranked in 16 days, and our ADC was telling me to play it safe, as if a couple weeks off from ranked means you're rendered useless. She then asked if I was B5, which I answered in the affirmative. This caused her to lament getting stuck with B5s when she's a B3 (B4 with 24 LP, in actuality). I ended the match with the most CS, a perfect KDA, and 3 towers to my name. And after the match, when I just wanted to put her in her place for doubting me, citing the same stats I did in the above paragraph, her response was just "cocky much?" This serves as a good example of how knowing your teammate's ranks, and nothing else, could hurt you more than help you. Clearly we all had approximately the same MMR, as that's how the matchmaking system works, but simply knowing I was B5 caused our ADC to write me off from the start. Riot doesn't want that sort of behavior.
So, yeah, just hop on OP.GG or any of the other sites that serve the same function and research your opponents that way. You'll get much more out of it, and you'll be less likely to misjudge someone based upon a rather meaningless rank.