I can suggest two ways one of the is if you have a polygon as a geometry in the map then you may use polygon.contains(point) as long as they are in the same spatial reference, it's a synchronous method, which means process wait until it completes. The other option is GeometryService.instersect([polygon], point, successFunction, errorFunction)
GeometryService allows you to intersect multiple geometries to a geometry in the same spatial reference. This method is a asynchronous method, which means the process will be done in the background. Once it completes calls the successFunction, if there is no error. I'm assuming the map service's geometry type is polygon. So another option I can think is Identify the point in the polygon. If it returns with results then the point intersects to the polygon.
var identify = new IdentifyTask(url);
identifyParams.geometry = evt.mapPoint;
identifyParams.mapExtent = map.extent;
identify.execute(identifyParams, successFunction, errorFunction);
I hope it helps.