D.B.
Don't worry you will know. She will do it on her own believe it or not. She will start taking her first morning nap later & her afternoon nap will get pushed back and then next thing you know she will be alert and crawling/running around not needing that last "late" nap. Sounds like she still needs three naps, so don't do anything right now. It will probably stay this way until 9-12 months. It's weird, but it happens almost overnight that they don't need the 3rd nap. Enjoy it for now. :)