Note: This text was written for women, but can equally be applied by men.
First of all, pray about it. Our Lord and His holy Mother are the primary source of aid for such matters.
Then ask yourself, can I see him as the father of my children? You should find that this narrows it down to perhaps one or two.
Now, imagine him as an old man with a wrinkled face and stooped over. Would you want to be at this man’s side? If so, then indeed there something of God’s hand in this relationship.
Next, observe him around girls or women with whom he is not interested in having a relationship, like a sister or feeble or old women. How does he behave around them? Here you will see his true character.
Finally, make sure he is someone who is fighting to overcome his passions and to correct his defects, no matter how large or small they may be. Make sure he is someone who is a true Christian.
Based on St. John Chrysostom, St. Paisios the Athonite, Father Dumitru Staniloae