I think I have this right now... 
Firstly, make sure the Ajax libraries are included in your file.
For your main PHP file, you need to have the following javascript/Ajax included:
<script>
        function add_source_reference()
        {
            $.ajax({
                type: "POST",
                data: {
                    source_ref: $('#source_ref').val(),
                },
                url: "gl_validation_checks.php",
                success: function(data)
                {
                    if(data === 'SOURCEREF_EXISTS')
                    {
                        $('#sourceref_res')
                            .css('color', 'red')
                            .html("This reference already exists");
                    }
                }
            })              
        }
</script>
You need to add an OnExit command to a text field (the "source_ref" field), where this equals "add_source_reference()"
Create a new PHP file called gl_validation_checks.php for example:
<?php
    $source_ref = mysql_real_escape_string($_POST['source_ref']);
    $sql = "SELECT source_ref FROM bank_trans WHERE source_ref='".$source_ref."'";
    $query = mysql_query($sql);
    if(mysql_num_rows($query) == 0)
    {
        echo('NO_SOURCEREF');
    }
    else
    {
        echo('SOURCEREF_EXISTS');
    }
?>
Make sure you include:
<span id="sourceref_res"></span>
Including this will indicate where the message will show if there is a duplicate.
I hope this all makes sense.