Choosing which test depends largely on the age of the child. If a child is less than 2 years old, doctors usually conduct a blood test. But if a child is older than that, a skin test can be done. Both have disadvantages. Blood tests are invasive and are only as good as the lab that does the testing.
Many doctors do both.
Interpreting the test, however, is an art rather than a science. Parents need to keep an accurate food diary for 2 weeks. Before testing, most allergists recommend eliminating the suspected food for 2 weeks. It's important to learn how to read a label so the suspected food can be removed from a child's diet completely. If, after the elimination, the symptoms stop, then the allergy is determined.