Chris, It might be hard to do if you have little experience in looking at many examples of strawing. New strawing is very yellow and looks new. Old straw can cover the spectrum from in the white to like new yellow strawing. Like bluing I would look for patina, high wear spots and what looks like natural wear. This is a very thin layer and tends to wear pretty rapidly. It can be stripped pretty easily and re strawed in your toaster oven. If you screw it up it can be done over. I would find suspect almost any straw that looks too new in comparison to the rest of the pistol's condition. I have heard that sunlight weakens the straw color but I cannot say for sure.
Maybe someone that has much more experience like Thor(AKA Ted Green of NM, Ted's Strawing Service) can give us his opinion. Jerry Burney