One sign could be if your best friend is overly secretive around your partner. For example, they might always have private conversations that seem a bit too hush - hush. Another sign is if your partner suddenly starts acting differently towards you, like being more distant or cold, and at the same time seems to be getting closer to your best friend. Maybe they start having inside jokes that you're not a part of. Also, if your best friend always seems to be around when you're not there, like they're waiting for you to leave so they can be with your partner, that could be a red flag.
Well, if your best friend is constantly trying to undermine your relationship in subtle ways. Say, they keep bringing up your flaws to your partner when you're not around. And if your partner starts taking on your best friend's opinions over yours a lot. Like, if you two had a plan and then your partner changes it because your best friend said something different. It might also be a sign if your best friend is overly interested in your sex life with your partner and asks inappropriate questions about it, as if they're trying to find a way in.