Not 100% reliable, but another way to check for detailed maintenance is to go to owners.lexus.com, register for an account, then "add" the VIN of the truck you're considering to your list of vehicles. Note that if you already have an account at owners.toyota.com, the credentials are the same.
Once you've added the truck, take a look at the Service History. If the timing belt has been done by either a Lexus dealer, or by someone that manually added the maintenance to the database, then you know it's been done, regardless if there's a sticker there or not. But if it's not there, and no sticker anywhere under the hood, it still doesn't really mean it HASN'T been done, just that they left no records.
As someone that is searching HARD for a GX now, I find this tool invaluable. Trying to find one with all the things I want is very challenging. I'm gonna have to travel.
