This question already has an answer here:
HTML Code
<div class="parent1">
    <div class="child1">Child1</div>
    <div class="child2">Child2</div>
    <div class="child3">Child3</div>
</div>
<div class="parent2">
    <div class="child1">Child1</div>
    <div class="child2">Child2</div>
</div>
jQuery Code
alert($(".parent1").find(".child3").text());
alert($(".parent2").find(".child3").text());
My question is how to check find function return true or false? 
In above code it return blank value, where parent1 class have child3 class where parent2 class do not have child3 class.
You couldn't use just find in if condition. You could use has or check for the length property.
var elm = $('.parent1');
if(elm.has('.child3')){
   var child3 = elm.find('.child3');
}
Or simply like this
var child3 = $('.parent1').find(".child3");
if(child3.length > 0) {
  // child3 is present
}
                    
                                            has doesn't work for me - mario1ua
You can use .length property to check that element exists.
var elem = $(".parent2").find(".child3");
if(elem.length){
    alert(elem.text());
}else{
    alert('Element doesnt exists')
}
Alternatively, You can use .has()
var elem = $(".parent2");
if(elem.has(".child3")){
    alert(elem.find(".child3").text());
}else{
    alert('Element doesnt exists')
}
                    
                                            
Whenever you wrap a selector in $() it will always return a jQuery object that contains the array of matching elements.
Therefore you can use the length property to test if there are matches
var $collection = $(".parent2").find(".child3").length;
if ($collection.length)....
You can use other approaches also such as is()
var #collection = $(".parent2");
if($collection.children().is(".child3") /* returns tru if there are matches
                    
                                            
As the docs for find() explain, find() returns a jQuery object. It has a length propery which can be checked for a successful query.
if($(".parent1").find(".child3").length > 0) {
    alert("parent1 child3");
}
if($(".parent2").find(".child3").length > 0) {
    alert("parent2 child3");
}
                    
                                            
There is no element .child3 for .parent2,
alert($(".parent2").find(".child3").text());
Try this:
alert($(".parent2").find(".child2").text());
Try this if you want only last item
alert($(".parent1").find("[class^=child]:last").text());
alert($(".parent2").find("[class^=child]:last").text());
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="parent1">
  <div class="child1">Child1</div>
  <div class="child2">Child2</div>
  <div class="child3">Child3</div>
</div>
<div class="parent2">
  <div class="child1">Child1</div>
  <div class="child2">Child2</div>
</div>
console.log(Boolean($(".parent1").find(".child3").length));
console.log(Boolean($(".parent2").find(".child3").length));
    
console.log(!!$(".parent1").find(".child3").length);
console.log(!!$(".parent2").find(".child3").length);
console.log($(".parent1").find(".child3").length > 0);
console.log($(".parent2").find(".child3").length > 0);
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="parent1">
  <div class="child3"></div>
</div>
<div class="parent2"></div>
finddoesn't return a boolean value. - undefined