You typed “mobiles” when you said you wanted to check for post type “mobile”. So simply remove the s ;).
<? if( post_type_exists( 'mobiles' ) ) { echo 'The Products post type exists';} ?>
Should be:
<? if( post_type_exists( 'mobile' ) ) { echo 'The Mobile post type exists';} ?>
If it’s still not working, triple check if you succesfully registered a post type named “mobile”.