I'm not sure I understand the spirit of your question. My apologies of this answer is obvious to you and you intended something else.
Simplify::badSet = "Function Set used in assumptions where is likely in error.";
SetAttributes[mySimplify, HoldAll];
mySimplify[expr_, assum_] :=
Message[Simplify::badSet] /; ! FreeQ[Unevaluated@assum, Set]
mySimplify[else__] := Simplify[else]
Now:
mySimplify[x + y, x = a]
Simplify::badSet: Set used in assumptions where is likely in error. >>
You could overload Simplify itself if you are comfortable with such things. You'll want to be familiar with the Villegas-Gayley method of you choose that path.
