I'm not sure that there is a right or wrong answer to this question. I think it depends on the circumstances. Obviously, I could say that the topic of marriage on a first date or first "getting-to-know-you-phone-call" is too soon, but anything is possible with God. I've always heard that Christians date to marry...meaning we, as Christians, don't just date around out of boredom or loneliness. With that said, my husband and I started talking about marriage early into the relationship. The backstory on that is that we'd known each other for six years and had dated before. We had a lot of history together...good and bad...it was assumed that if we were to start dating again it wouldn't be just for kicks.
To answer your question specifically, I do think that you can know whether or not someone is your spouse early in the relationship. Continue to seek God. Really, really seek God.