Red flags: talking badly about other people often or all the time.
Snide remarks.
Bitchiness.
Accepting lots of favours but never offering help.
On the other hand I also have no old friends. 
I have moved a few times but more importantly, the friends I made over the years have been often quite toxic and sometimes not 'relevant' as both parties develop and move on.
My first BFF from school was lovely but very intense. It was more like having a bf and she ended up copying everything I did including training in the same profession as me, even contacting my employer to ask them questions about how to succeed in the industry
.
I moved country and my next 'BFF' was someone from my home country and we gave each other a lot of support whilst away om home. But we really had nothing in common other than a shared background and, whilst living together in a flat share in London, she started hanging out with a bunch I do didn't like. They did a lot of coke and other stuff which was not by cup of tea.
In the meantime another value friend of mine back home had a depression and I supported her remotely and stayed in contact for another give years. A that point she started having lots of affairs with married men, which put me off. The friendship drizzled out.
Then.... A few years after when I was 30 I became very close fronds with a woman and our dh became very close. She turned out to be the umber manipulative scheming narcissism person and it took me 3 years to figure out that this was going on. I neatly lost everything through her but only had been supportive, calm and reliable with her. Since then I have become very very wary and am pretty afraid of making friends.
I didn't just have besties during this time. Sadly it seems that the last 'old' friend I had who never was a 'BFF' but always a good friend seems to have grizzled out as I have married, moved out of London and have dc one with additional needs. She is single, parties a lot and it seems to have grizzled. I am very sad about this. Anyways not sure if I am the root cause of all these failed friendships but my history is not brilliant.