How can I tell if a G430 or G1000 has WAAS or not?
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.