The clearest indication that your girlfriend is in the mood to make out is if she initiates a make out session with you, or if she asks you to make out with you. However, there are other, less direct indications as well.
Many people will overtly flirt with you, vocalize sexual innuendos, and become physically excited (also known as "feeling horny") when they want to be sexual with you.
Although none of these indirect signals are sure-fire signs that your girlfriend wants to make out with you, they are general indications that you can use to guide your actions.
Regardless of what indirect signs you are getting, you should be aware that a verbal indication to stop, such as hearing "No," means that you should stop altogether (or at least slow down, and revert to your previous activity). Failing to respect your partners' consent is legally considered rape.