[fen "rn1qk2r/pbp2ppp/1p1bpn2/8/2P5/5NP1/PB1PPPBP/RN1Q1RK1 b kq - 0 8"] [White "Villain (2075)"] [Black "Hero (1950)"] 8... c5
I was Black in this position. To me, it immediately looks like the position calls for 8...c5: I need to prevent White from taking over the center, I need more space for my pieces, and I need to protect my queenside from attack on the long diagonal h1-a8. My plan was to play c5 and Qc7 to target the center with all my pieces.
However, after considering the move, I thought that it does not help me much: after I have played for example c5, Qc7, and Nc6, I didn't see any possible continuations. My bishop is going nowhere on the d6-h2 diagonal, and my knights have no good squares nearby if white protects d4, so I would probably need to continue with 0-0, Nfd7 and f6 to put something on e5 to get any control of the game. I also dreamed of building an attack with something like h7-h5-h4 but decided that it takes way too long and white has too many ways to defend.
So I decided that I really need to keep the c5 square free in order to have the choice to play Nc5 or Bc5 (or even Bb4 at some positions) later.
However, the queenside still is a problem, so I ended up playing 8...c6, which allowed 9.Re1 threatening e2-e4-e5, forcing me to block the e5 square with my pawn, resulting in a loss after 12 moves of misery (apparently 9.e4 would have been even better).
After the game, my opponent said that I should have played c5, but didn't say anything after I mentioned that I thought it restricts my mobility too much. Was it really a good move? Was it the least bad idea in this position? Is there something better? How can I know the answer during the game without calculating every variation?