The National Association for Gifted Children is one of the most authoritative sources on gifted children and education (see first link below). They have information for educators and parents on a variety of gifted topics. I reviewed their content on assessment and identification of children for giftedness, and I have included a link to their brochure for educators below.
 
In general, it appears that there is no "valid" 5-10 minute evaluation for giftedness, as the standards for giftedness evaluation include an emphasis on assessing a wide diversity of talents. That is, giftedness is not just how well a student can perform on a test of algebra. A true assessment of giftedness would include a variety of testing tools on various dimensions of educational ability (and, in some cases, artistic ability!).
 
That said, you can certainly use your instinct to evaluate a child's overall potential. If you believe that you have a student who has gifted potential, and your school has a gifted program, you should refer them for formal testing. I have included some additional links below from Hoagies that describe some of the recommended tests for high potential.
 
Good luck!
 
L. Compian, Ph.D.
Counseling Psychologist
Education.com Expert Panel