I have been seeing a girl casually for a while and I don't know if I am doing my best. How will I know she has come during sex and is satisfied?
First off, orgasming and pleasure are two different concepts and one does not guarantee the other.
Orgasm does not equal to satisfaction. It's a momentary release which feels good, but like I said, lasts only for a few seconds to minutes. Answering the first part of your question- The only way you can know if the person you are with has orgasmed is by asking them. There is literally no other way you could know if they climaxed or not. If you feel awkward asking this question directly, you can reframe it in other ways. For example- " This (describe or mention the act/moment) felt incredibly amazing and made me orgasm too. How about you? " OR " I could feel myself peaking to orgasm when you/I did (describe or mention the act/moment) " Try to make it sound conversational instead of a mechanical interview.
Getting to the latter part of the question, when we talk about satisfaction and pleasure, unlike orgasm, it's not restricted to a momentary release. Instead, it takes the entire experience and act in account. You can communicate and discuss the act in aftercare (cool-down phase after the sexual activity). You could start the conversation by saying how or what felt the most pleasurable to you (act, area, moment, etc.) and ask them about theirs. If they or you didn't find something pleasurable, ask them on what went wrong or what you would both like to try together the next time.
Advice: Focus on pleasure and let the orgasms follow. Thinking about orgasms and orgasming makes the entire act goal-oriented, mechanical and unnecessarily pressures all parties. Take it at your pace and focus on enjoying and experiencing all the sensations you can in this to intensify the orgasm.