It seems to me that we can assume that Romeo is not truly in love with Rosaline because his feelings for her vanish immediately when he sees Juliet. If he really, genuinely loved Rosaline, then he would not be able to develop feelings for Juliet so quickly. It would take some time for his feelings of love to resolve before he could love someone else. This is precisely what Friar Lawrence seems to refer to when he chastises Romeo, saying,
Young men's love then lies
Not truly in their hearts, but in their eyes. (2.3.71-72)
The friar essentially implies that Romeo did not truly love Rosaline but lusted after her only, experiencing feelings for her because he wanted to sleep with her and not because he actually felt love. One could charge Romeo with a similar accusation regarding Juliet. Is it possible that he could truly fall in love with her the first time he sees her? Of course, people often refer to the concept of love at first sight, but is it really love, or something else that they feel? Is this lust too? For those who doubt that Romeo truly loves Juliet, I think this line of reasoning is the most likely explanation.