In Romeo and Juliet, where are some key examples that prove the two are in love, or, on the other side, examples that prove they only feel lust?

Shakespeare's Romeo and Juliet is a tale of both erotic and spiritual love.

Prior to their encounter at the Capulet's celebration, Romeo speaks of Rosaline in the words of courtly love. Because he is so distraught at her rejection, Romeo agrees to accompany Benvolio only so that he can see Rosaline again. For, when Benvolio suggests that when Romeo goes along, he will see other girls who seem better, Romeo replies,

I'll go along, no such sight to be shown,
But to rejoice in splendor of mine own....  (1.2.102-103)

Romeo has been rejected by the chaste Rosaline, who does not return his desire.

In his first encounter with Juliet, Romeo cannot wait to touch her. In the sonnet which comprises their conversation, Juliet speaks seven lines and Romeo seven others, and the eroticism is apparent:

ROMEO My lips, two blushing pilgrims, ready stand
            To smooth that rough touch with a tender kiss.
JULIET  ...saints have hands that pilgrims' hands do touch,
            And palm to palm is holy palmers' kiss. (1.5. 91-96)

Unlike the unapproachable and chaste Rosaline, Juliet responds quickly to the passion of Romeo, making no apology for her carnal desires.

Further, when Romeo hurries to Friar Laurence in order to ask him to quickly perform the marriage ceremony for Juliet and him, the Friar cautions against such passion as Romeo exhibits. 

These violent delights have violent ends,
And in their triumph die, like fire and powder
Which as they kiss consume. (2.6.9-11)

Later in the play, Juliet demonstrates her true love for Romeo when she cries over Romeo's banishment:

NURSE: Weeping and wailing over Tybalt's corse. 
            Will you go to them? I will bring you thither. 
JULIET: Wash they his wounds with tears. Mine shall be spent.
            When theirs are dry, for Romeo's banishment. (3.3.95-98) 

In the final act, Romeo and Juliet spiritually love each other so much that they are willing to die for the other. Romeo, upon finding his beloved Juliet dead, says, "Well, Juliet, I will lie with thee tonight." (5.2.36)

Prior to this, in Act IV, Juliet takes the potion that she fears could kill her, but she does this in the hope that she can reunite with the banished Romeo if the Friar's plan works: "Romeo I come! This do I drink to thee." (4.3.58)