There is no way to guarantee that you are O'ing. If you are getting regular AF, you can be pretty sure that O is happening pretty regular. Every woman can have a slightly wonky cycle from time to time - it is normal, so there is never any guarantees.

Renstar - Temps drop just before O and rise after O. The temp drop cannot always been seen on a chart because it is a 12 hour drop in temp (up to about 0.3C), whereas temps are taken every 24 hours. A temp rise only confirms that O has taken place and it is common for this to be up to 3 days later. Generally, I find the temp rise to be about 0.5C. Are you using a mercury O basal temp thermometer? I generally found them to be more accurate.