How can I tell if a G430 or G1000 has WAAS or not? Both the GNS430 and G1000 are/were available with and without WAAS, and I believe it's even possible to upgrade a GNS430 to add WAAS support. When I get into an unfamiliar rental aircraft, is there a quick and easy way to see if WAAS is available, by checking the avionics themselves? Is anything physically different about the units, is there some properties/status page I can check, or is there something different on the display?

Obviously asking the operator and checking the aircraft handbooks would be the right thing to do, but sometimes that doesn't work out, e.g. the FBO's instructors are all out flying and the admin people just don't know.
NOTE: as pointed out in the comments, none of these quick verification steps actually guarantees that the airplane you clambered into is certified for LPV approaches, or for IFR GPS usage at all!

On boot, a 430W will display that it's a 430W:



Aside from that, I'm sure there's a version info screen somewhere (on both G1000 and GNS430), but the best solution I can think of is to check the satellite status screen. I'm trying to find examples of a 430W displaying it, and failing, but on a G1000, you'll see this:

 (source)

If the G1000 supports WAAS, it'll display that under the 'SBAS Selection' header and you'll see satellites at the bottom numbered in the 100s - 124, 126, 129, etc - all the 100-series satellites are the WAAS augmentation satellites. The satellites augmented by WAAS will also have a "D" at the bottom of the signal strength column.