EDIT: grr...darn you Jim...you and your fast typing fingers! Sorry for the duplicate info here.
Hey Bob,
That would be part of your research process.
You can call and get the service history on a vehicle by the VIN, or you can even set up an Owner profile at Lexus.com, add the VIN and look at the service history yourself.
The biggest blanket exclusions on warranty are when the vehicle is salvaged. Check the VIN with Carfax or Autocheck or preferably both. If the car has been issued a salvage title, they will typically have this information.
When I am buying, I typically assume that the warranty is valid, but I look for the service history and title status to confirm that there isn't a compelling reason why they would void the warranty. Remember that the Magnuson-Moss Warranty Act requires a manufacturer to prove that a lack of maintenance caused the failure and hence voided the warranty.
What you want to look for is that proverbial cream-puff, owned by the lil old lady who barely drove it and took it religiously in for service at the dealer.
You are making an investment. Invest in a vehicle that has clear proof of service and preferably a single owner. These cars are out there and the pricing is better than they have been in years. Enjoy the experience!