How can you tell very early if you’re just a back up plan?

This guy that I like has a female best friend that he has known for 2 years and whom he spends a lot of time with one on one. I’m pretty sure he has a crush on her but I still see him staring and flirting with other girls. I’m hesitant to approach him because I’m afraid to be only the backup in case he and his female best friend don’t work out. Are there any early signs that indicate you’re just used as a distraction or consolation prize?

How can you tell very early if you’re just a back up plan?
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