hook

Add this code to your functions.php at the theme you are using

function userIsInUamGroup($sGroup)
{
    global $oUserAccessManager;
    $oAccessHandler = $oUserAccessManager->getAccessHandler();
    $aGroups = $oAccessHandler->getUsergroupsForObject('user', $oUserAccessManager->getCurrentUser()->ID);

    foreach ($aGroups as $oGroup) {
        if ($oGroup->getGroupName() === $sGroup) {
            return true;
        }
    }

    return false;
}

Now you can use the function userIsInUamGroup to check if a user is in a group. Note that will not work for admins because these user have always full access and so the have no groups, so this function will return false.