BeginPackage["FunctionAnalysis`",{"Global`"}];
MonoIntervals::usage="returns monotonic intervals of given function";
Begin["`Private`"];
FunctionAnalysis`Private`MonoIntervals[func_[arg_],var_]:={Reduce[D[func[arg],var]>0,var]}
End[];
EndPackage[];

MonoIntervals[Sin[x],x]

f[x_]:=x^2+2*x-10
MonoIntervals[f[x],x]

Out[25]=MonoIntervals[-10 + 2 x + x^2, x]