An infatuation is an intense passion felt in a short moment of time. It's not a long lasting feeling. In contrast, real love runs much deeper than infatuation. Real love is a genuine admiration for a person's qualities and character traits. While the same person may also have a few vexing character traits, real love looks beyond the little things. Real love is more than just a feeling; it is actually a conscious choice made to continue valuing the person, to continue to be patient, gentle, understanding, and faithful. Romeo and Juliet's relationship can certainly be characterized as infatuation rather than real love as neither one of them really knew each others' character traits well enough to develop an admiration for each other that ran deeper than physical attraction.
One piece of evidence we have of their infatuation, especially Romeo's infatuation for Juliet, is that the first moment he sees her he speaks only of her beauty. He compares her to a bright jewel, like a diamond, on an Ethiopian's ear. The brightness of the diamond would stand in great contrast against the darkness of the Ethopian's skin, showing off the diamond's beauty. He then refers to her beauty as "[b]eauty too rich for use, for earth too dear!" (I.v.49). He then compares her to a holy shrine as if her beauty alone is enough to make her a virtuous person with a strong character. The most telling line of all that Romeo is confusing real love for infatuation is that he asks himself if he has really known love until now, but then goes on to state that his love is being guided by his eyes and argue that of course he never knew love until this moment because he never really saw true beauty until this moment, as we see in his lines, "Did my heart love till now? Forswear it, sight! / For I ne'er saw true beauty till this night" (54-55).
Even Friar Laurence rightly characterizes their love as young infatuation when he accuses Romeo of being fickle and declares that "[y]oung men's love then lies / Not truly in their hearts, but in their eyes" (II.iii.68-69). He even calls Romeo's love young in a metaphor personifying love as being too young to really know how to read.
Hence, since both Romeo and Juliet are clearly very young, hardly know each other before they declare their love for each other, and focus on physical attraction, their love is clearly infatuation rather than real love.