$(".form-dotaz").submit(function(){

  var ok = true;

  if($("#jmeno").val() == ""){
  
    ok = false;
  }
  
  if($("#email").val() == ""){
    ok = false;
  }
  
  if($("#text-dotaz").val() == ""){
    ok = false;
  }

  if(ok){
    return true;
  }else{
    $(".form-error").fadeIn();
    return false;
  }

});