In Act 4 Scene 5 of Romeo and Juliet:Do Lord and Lady Capulet really love Juliet? How do you know this?

Yes. First of all, there is a reason, other than a selfish one, that Lord Capulet has acted in haste to have Juliet married; Paris informs Friar Laurence in Act IV,

Immoderately she weeps for Tybalt's death,

And therefore have I little talked of love,...

her father counts it dangerous

That she doth give her sorrow so much sway,

And in his wisdom hastes our marriage,

To stop in inundation of her tears,

Which, too much minded by herself alone,

May be put from her by society.

Now do you kow the reason of this haste. (IV,i,6-15)

So, Lord Capulet has Juliet's interests in mind when he insists upon the wedding with Paris.  However, when Juliet protests the marriage, arguing that she is too young and she cannot love, he becomes incensed that she should defy his will.  Yet, given the fact that he is a aristocratic patriarch of the fourteenth century, his anger at being defied is not unusual:  Capulet simply cannot understand Juliet's disobedience.  Later, after Juliet meets with Friar Laurence, who devises a plan for her, she returns to her father and tells him that she will wed.  With this news, Capulet is delighted, exclaiming,

....My heart is wondrous light

Since this same wayward girl is so reclaimed. (IV,iii,44-45) 

These words of Lord Capulet indicate that he is happy because Juliet has obeyed him, and also because she has returned to him as his sweet daughter.  There are indications here that Lord Capulet loves Juliet.

Then, after Juliet appears to be dead, Capulet is very distraught:

Death is my son-in-law.  Death is my heir,

My daughter he hath wedded.  I will die.

And leave him all--life, living, all is Death's.

After Paris arrives, Lord Capulet says,

O child!  O child! My soul, and not my child!

Dead thou art!  Alack, my child is dead,

And with my child my joys are buried! (IV,v,52-55)

That Lord Capulet has placed his happiness and his soul in the life of his child gives a strong indication that he does, indeed, love Juliet.

Likewise, as Lady Capulet learns of her daughter's death, she exclaims, "Oh, woeful time!" and calls lovingly to Juliet, "My child, my only life" (IV,v,13), and tells her daughter to "revive" or "I will die with thee" (IV,v,14).  As Friar Laurence and Lord Capulet speak, Lady Capulet laments,

Accurst, unhappy, wretched, hateful day!                        

Most miserable hour that e'er time saw (IV,v,33-34)

Lady Capulet, like her husband, is clearly distraught at the death of her only child, exclaiming that she, too, will die as Lord Capulet has also said. Both parents feel that life is not worth living without their child.