the direct answer to your question is

test="following-sibling::*[1][self::header]"
which is true just if the immediately following sibling is header, but
it sounds  lie wha you really want is to group the child elements of
root, with each group starting with a header, so that's

<xsl:for-each-group select="*" group-starting-with="header">
  ... something ...

David