I used to fly on the west coast corridor more than 3 RTs a week. I used to do a lot of standbys and as an EXP never had problems except on Fri afternoons (they were OK after 7 PM on Fri).
You cannot check availability by looking at seatmaps but can guess it by using itn.net (not too useful these days) or similar sites:
http://www.etour.co.jp/Solar/index.html - Japanese site discussed in FT
http://www.aeroplan.de/ap_rb_galileo.htm
Galileo site discussed again in FT
If you see a lot of availability in cheap fare codes (L, M, N, V, etc.) you can reasonably expect your standy to clear.