Esselamu aleykum sisters and brother
I know that if a man is not mahram to you , you are not allowed to show your hair.because he is not your husband on that time.
but people should see and talk with each other between the frames of halal while they have an intention to marry.
Generally I see that in practice , these boys mother check the girls hair or whatever,
because what if she hasnt got hair !!
it is narrated from Imam Muhammad Nur Abdullah of St. Louis, Missouri:
"When one of you asks a woman for marriage, if he is able to look at what will induce him to marry her, he should do so...." (Abu Dawud).
This means the two potential spouses can look at each other but not ogle or stare. Abdullah also noted that there is no limit on the number of times the two people can look at each other. However, both should fear Allah and remember the purpose of this is to satisfy the need for physical attraction to the person you are marrying. He also notes it is not permissible for a man to see a potential wife without Hijab, since he is not her Mahram (a relative with whom marriage is not possible, or legally her husband). Abdullah says seeing her face and hands are enough to determine attraction.