You'll be able to tell your woman is coming when:

she tells you: she moans one hell of a lot and her breathing changes
she flushes: over her face, neck and/or chest
her vagina begins to flutter rhythmically around your finger

You may see all of these things, or you may see none of them. If you miss an orgasm, don't stop unless you or she wants to. Women have startling recuperative powers, particularly when they're receiving the right level of attention, and generosity is its own reward. Multiple orgasms are not mythical.

Once you believe that your woman is coming you should shift your attention from the clitoris to the G spot. Keep up the same rhythm, but use more pressure. You may want to keep some sort of contact with the clitoris, but just as a penis becomes super-sensitive during orgasm, to the point of discomfort, so can a clit.

As with the clitoris, you should pay attention to whether the woman pushes towards you, draws away from you or just sits there to gauge the amount of pressure you're giving. You probably won't need to vary your speed much, but pay attention to what she says she wants.