Most people you will meet are what they say they are. The minority who are not tend to behave in recognisable ways, and the cost of recognising them late is high enough to be worth learning the patterns.
Pressure about time
Urgency is the most reliable warning sign there is. Someone who needs you to decide quickly, meet quickly, or conceive quickly is optimising for something other than the child. There is no version of this decision that is improved by speed.
Vagueness about the future
Ask a direct question about contact, money or legal status and listen for whether you get a direct answer. “We’ll figure it out” is not agreement. It is the absence of agreement, and it will surface later as a disagreement about what was always assumed.
Natural insemination
A donor who raises this, at any point, in any framing, is not offering donation. Every legitimate route to conception here is clinical or self-administered. There is no exception, no medical justification, and no reason to continue the conversation.
Reluctance to be verified
Every reason someone gives for avoiding identity verification is a reason that applies more strongly to you than to them. Privacy is a good argument for sharing little publicly. It is not an argument for being unidentifiable to the person you are asking to have a child with.
The quiet one: not asking
The pattern people miss is a donor who asks you nothing. Someone who will help create a child and has no questions about the life that child will have is telling you how much thought they have given it.
What to do
Slow down, put it in writing, meet in public, tell someone where you are, and take legal advice before anything happens. Nobody who is genuine will object to any of that.