After a look at matlabroot\R2013a\toolbox\rtw\rtw\tlc_c.m around line 176, the parameter 'RTWCGKeepContext' seems to hold during RTW builds and not to hold during normal simulations.
function Start(block)
if strcmp(get_param(gcs,'RTWCGKeepContext'),'on')
disp('### Detecting a RTW build');
% do not start the simulation locally
% why is this function called when the RTW model is being built?
return;
else
disp('### Detecting a normal simulation');
end
...
end
However, there should be a cleaner way to detect this, as in C S-Function.
