Only a small percentage of children with Autism Spectrum Disorders are diagnosed before they are three years old, although earlier detection and intervention would reduce the disabilities associated with the disorder. In addition, as children get older, treatments are more costly and difficult and the results less satisfactory. Considering the importance of detecting autism early, the objective of this study is to identify the instruments that can be used to detect signs of autism before children are 2 years old, and which of these instruments have been validated in the Spanish population. By searching in several research databases, we compared the existing instruments and their main characteristics. We found that some instruments can be used to assess a possible autism spectrum disorder before children are 2 years old, with acceptable sensitivity, specificity and reliability indexes. However, only a few instruments have been validated for the Spanish population, some of which have not been specifically designed for early detection. For this reason, a tool needs to be developed to detect the warning signs of autism spectrum disorders before the age of 2 which can be applied as part of the protocol for pediatric check-ups.