BOOL×BOOL→ENUM mapping
I'd use a decision table here:

GameState[,] decisionTable = new GameState[2,2];
decisionTable[0,0] = GameState.Ok;
decisionTable[0,1] = GameState.Stalemate;
decisionTable[1,0] = GameState.Check;
decisionTable[1,1] = GameState.Mate;
return decisionTable[Convert.ToInt32(isCheck), Convert.ToInt32(isStuck)];


From Code Complete 2nd Edition, Chapter 19: General Control Issues, page 431:


  Use decision tables to replace complicated conditions
  
  Sometimes you have a complicated test involving several variables. It can be helpful to use a decision table to perform the test rather than using ifs or cases. A decision-table lookup is easier to code initially, having only a couple of lines of code and no tricky control structures. This minimization of complexity minimizes the opportunity for mistakes. If your data changes, you can change a decision table without changing the code; you only need to update the contents of the data structure.