jqGrid supports varios rules for client-side validation. They are available through the editrules option for each column. Since this is a collection, you can stack several validation rules at the same time - they will all apply to the respective edit field.

For this particular example we will check if the entered Freight value is a number and additionally to this we will validate the number if it is little than zero.
For this purpose we will use the example 1 from here

<?php
require_once 'jq-config.php';
// include the jqGrid Class
require_once "php/jqGrid.php";
// include the driver class
require_once "php/jqGridPdo.php";
// Connection to the server
$conn = new PDO(DB_DSN,DB_USER,DB_PASSWORD);
// Tell the db that we use utf-8
$conn->query("SET NAMES utf8");

// Create the jqGrid instance
$grid = new jqGridRender($conn);
// Write the SQL Query
$grid->SelectCommand = 'SELECT OrderID, OrderDate, CustomerID, ShipName, Freight FROM orders';
// Set output format to json
$grid->dataType = 'json';

//set a table to be manipulated
$grid->table = 'orders';

// set the primary key - it is serial
$grid->setPrimaryKeyId('OrderID');

// Let the grid create the model
$grid->setColModel();
// Set the url from where we obtain and edit the data
$grid->setUrl('grid.php');

//do not allow this field to be editable
$grid->setColProperty('OrderID',array("editable"=>false));

//set validation rules for Freight
grid->setColProperty('Freight'=>array("editrules"=>array("number"=>true,"minValue"=>0)));

// Set some grid options
$grid->setGridOptions(array(
    "rowNum"=>10,
    "rowList"=>array(10,20,30),
    "sortname"=>"OrderID"
));

// enable form editing
$grid->navigator = true;

$grid->renderGrid('#grid','#pager',true, null, null, true,true);
$conn = null;
?>