Unless you have super hearing you're not going to detect any change in a man or woman's voice. The key is body language. I work for a car dealership and our salespeople can tell if a customer is serious about buying just by looking at them. All of our salespeople are full time, we don't do part time, weekend sales bullshit. Body language is the most easily detectable in just about every environment.
pop an Adderall and read this https://www.amazon.com/What-Every-BODY-Saying-Speed-Reading/dp/B006ZNFEKW
I read What every BODY is saying by Joe Navarro, this book is pretty darn detailed. I liked it a lot.
http://www.amazon.com/What-Every-BODY-Saying-Speed-Reading/dp/B006ZNFEKW