They excessively boast upon first meeting them.
They are excessively friendly.
Do they have unhealthy relationships with their family?
Do they follow through with their promises, even on minor matters?
How do they treat unfamiliar individuals, such as the waiter or the server?
Do they tip poorly?
Do they hold the door for others or let it close on them?
Observe for passive-aggressive behavior as it often conceals other issues.
Are they unreliable? Do they cancel plans frequently?