IsPlayerInACL

Code

function isPlayerInACL(player,acl)
	local account = getAccountName(getPlayerAccount(player))
	if(isObjectInACLGroup("user."..account,aclGetGroup(acl))) then
		return true
	else
		return false
	end
end

Usage:

function playerLogin()
	if (isPlayerInACL(source,"Admin")==true) then
		outputChatBox(getPlayerName(source).." has logged in!", root, 0, 255, 0)
	end
end
addEventHandler("onPlayerLogin",root,playerLogin)

Code by xXMADEXx