Possible ways:
1. College application. Yes, the checking the box is the final thing to do if you haven't taken any other steps to get on the multicultural radar.
2. College websites. When you sign up to get additional information, they often ask about ethnicity & race.
3. College diversity offices. Many schools have special offices that help recruit URMs, eg.
Student Recruitment Coordinators | Multicultural Outreach Program | Office of Undergraduate Admissions
4. National Hispanic Recognition Program. Several students on this board did not mark Hispanic on their PSAT, but when their scores came out, it looked like they would qualify as a Nat Hispanic Scholar. They contacted NHRP via their HS GC and were recognized. See the "Resources" sticky thread on this forum for links to information about NHRP.
5. Ventures Scholar. If the student is interested in math/science, they could join this organization, see the "Resources" sticky for a link. This organization also provides names of admissions personnel for participating colleges:
Ventures Scholars Program | Undergraduate Admissions Personnel
6. Diversity weekends. See the "Resources" sticky for a link to a thread discussing fly in possibilities.
7. Summer programs for URMs, see the "Resources" sticky.
How much being a URM will help depends on each individual situation. For instance, factors that affect how much it will help include: statistics (gpa & test scores), rigor of coursework, the rest of the application (essays, LORs, ECs,), HS (eg, private or suburban vs. underserved), SES, identification with ethnicity, overcoming adversity, etc.