Instead of checking for ModelState is valid or not in every action, encapsulate this check in a Action Filter.

ModelStateActionFilter.cs

public class ValidateModelAttribute: ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        if (filterContext.Controller.ViewData.ModelState.IsValid)
        {
            return;
        }

        filterContext.Result = new ViewResult
        {
            ViewData = filterContext.Controller.ViewData
        };
    }
}