Minor pieces -- whose are better. Are you willing to trade a knight for bishop? Be sure to look at what's left on the board not what's being traded. Closed games challenge amateurs to remember their worst pieces. While most amateurs like to play with their best pieces, in closed positions you should spend time and concentration on your bad pieces to find good future squares for them. e.g. a quiet Bd7 in a French for black for an easy example.
Figure out what side of the board you'll be seeking play on (generally where you have more space then your opponent i.e. where your pawns "point" a la Silman.
Watching to see if the opening of a position (by your or your opponent) favors either player. A good rule of thumb is that an opening favors whoever is better developed -- or in the case of a middlegame who is better deployed in that area. If you have pressure on the c file with your rook(s) and his queenside is undeveloped/most his pieces are on the kingside, then you can be sure opening lines on the queenside will help only you.
Look for permanent weaknesses or targets in the pawn structure. If he's weak on the light squares, playing b4 to fix a black pawn on b5 is an example of fixing a weakness.
Hmm, that's about it off the top of my head.
I've classically been an e4 player that plays sharp variations against the sicilian and italian game against e5. I've been trying to play more d4 but this results in more closed positions that I can't prevent.
What things should I be thinking about in a closed position at the level I'm playing at (2000+ chess.com)