You should be able to see the arguments passed to dmcs in the Build Output when you compile. The Build Output is inside the Error List window. Select View | Pads | Error List, then click the Build Output icon.
For a release build typically the two arguments that make it a release build are:
/debug- /optimize+
Mono's C# compiler has similar arguments to the Microsoft's C# compiler so the information on the MSDN could be used to understand what these arguments mean.
If you are using xbuild to compile your project then you may not see the arguments being passed. However you can drop down to the command line and run xbuild from there to see the arguments being passed:
xbuild YourSolution.sln /t:Rebuild