op..what I mean by 'there' is what somebody has said up thread, the chemistry,spark,whatever you want to call it.
When I was OLD, some years ago (and I did a fair bit) I made the mistake (repeatedly in the beginning) of chatting on the phone,texts, and even MSN messenger..it was that long ago! Well it was disastrous. A bloke I'd chatted to (phone) for 6 weeks, had a lovely voice and a dry wit, I was smitten..a huge letdown in RL I'm afraid, no spark, nothing, couldn't understand how that could happen.
Unfortunately it is all too common, so I'd say, meet at the first opportunity, don't leave for weeks and don't keep texting/chatting first.
As for red flags hmmm...just ask loads of questions, kind of like an informal interview (making mental notes) crikey..am I bad or what!
Go and enjoy your OLD experience..worry about the red flags when it's 'there'..