Alright, Having gone through this process a great many times for all kinds of apps let me give you some advice. First of all Hispanic/Latino is NOT a race so you may mark Hispanic or Latino regardless of whether you then proceed to mark Black, White Asian or whatever. On the common app the question is Are you Hispanic OR Latino. To that question you should answer yes. If you are Caucasian, mark white, if you are Mestizo or of a Native South American tribe mark Native American and clarify it. That's what I did.
Now as to whether or not you will be considered a URM, that is completely arbitrary based on a college and may or may not give you an edge depending on everything from program choice to University choice to your own recognition and collection with your heritage.
Having said all that remember to answer as CLEARLY and carefully as you can. We were told a story at a summer program at Dartmouth about an applicant lying and generally if colleges have reason to believe something is dishonest or misleading it will probably lead to an auto reject without strict verification of the issue. If you had 40,000 apps and there was a student you felt was being dishonest you would not have the time to check, but it would impact your admission decision all the same. If you have previously always marked white without identifying as Hispanic this is an example of one such discrepancy which may give the impression of being dishonest, be it true or false. Be honest, be straightforward and don't be worried about whether or not you'll get an edge. If you are admitted it will have a lot more to do with who you are in school than who you are in heritage, race or nationality.
If you reported white and did not mark Hispanic on your SAT/ACT or your high school you would do well not to change this on your application now. Inconsistency may go completely un-noticed but it may also earn you a negative, which is hardly worth the slight edge you may or may not actually be getting from it.