The Verifications XSD is located here.
Your favorite IDE probably supports validation and/or syntax highlighting for XML files by using XSD schemas. For this to work you need to specify the schema in your verifications.xml file:
<verifications xmlns="http://maven.apache.org/verifications/1.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/verifications/1.0.0 http://maven.apache.org/xsd/verifications-1.0.0.xsd">
...
</verifications>