You get that feeling of tranquility. That feeling that the search is over. The need to keep looking vanishes. I couldn't imagine someone being more perfect, so why would I keep looking? I said it in one of your other threads as well, but here it is again:
when you find someone who understands you with a single word, accepts and even loves your flaws, is able to see right through you and not run for cover, and gives you the benefit of the doubt...why would you let them go? I could see myself growing old with him. Others, I felt anxious, thinking about still being theirs a year, 5 years, let alone 10 years down the road.
FWIW, I am pretty sure I'm there right now, and I haven't popped the question yet (looking into buying the ring--she knows, since I asked her about her ring size
) So my perspective might be great for the OP.
The obvious sign is the fact that I feel completely tranquil and excited at the same time around her. It doesn't take much compromising for me to be with her, or what little compromising it does take I'm perfectly fine with. We recently went on a camping trip with friends and everyone else says the trip was a disaster, mainly due to the drama around everyone, but I left that weekend with her feeling like I just had the best weekend of my life. The drama rolled off my back because I didn't care--I just went on a walk or hike with her and remained blissfully ignorant of everything, and she did too.
The rational signs line up when I consider our life situations and how we complement each other with our experience, with our skills and outlooks to the point that I can imagine life with her going forward will be extremely full of opportunity and fun, and life without her will leave me feeling sad and desperately regretful, considering what an incredible life I could have had I stuck with her. She's an amazing catch. From what my friends tell me, I am too, and she knows this. She has amazing insight into people (best guess ISFJ here), and I guess she knows the "type" of person I am and what folks like me are capable of. I supposedly personify most things she wants in a man, and it was actually a conversation about that with her coworker that lead to us meeting (her coworker, a friend of mine, introduced us).
We've moved fast, and it seems we've moved through at least one major relationship "landmark" every 2 weeks since we met (we've only known each other for, eh, 77 days as of today...)
And we had a good talk about this last night, and we're both extremely comfortable with how fast things have been moving. As she put it, "what's the point in holding things up?"
So yeah, it's been a constellation of emotional and physical chemistry, our respect and admiration for others' life experiences and life skills, our mutual desire to start a family and have kids (her time is running out... that's one reason I think she is letting things move so quickly), and the final "do I ever want to live without her?" question whose answer tells me NO, I never want to live without this woman. EVER.
(And as a final note of verification, some of my friends went through this kind of fast "we just knew" experience themselves before getting married... and they agree this looks exactly like what they went through)
Also, I just "knew" something was going to happen the first night I met her. I was informed about her a little in the month leading up to that night, but it was raw chemistry that got my jets goin'. That glance across the table, with the nervousness and profuse sweating (on my part anyhow)... had me intoxicated for the next week.
When we went out and he could quote the entire Holy Grail, that was it. I was having his babies. 
