Do you mean pages which have AMP as their only output format (or don’t have HTML as their format)? You can check for existence of HTML output format:
{{ range .Data.Pages }}
{{ if .OutputFormats.Get "html" }}
...
{{ end }}
{{ end }}
I don’t know of any convenience functions to filter them directly or with where pipeline.
From my experience, if you have both HTML and AMP as output formats, the sitemap shows link for the default one (not AMP), so you should have no problem with those.