Here is an infrared image of part of the fragment--look how beautiful this is!--from שירת הים, from here:

Note the sirtut; note the absence of tajjin (not me'akev) and the form of the letters. So, how do they know it's a Torah scroll and not a chumash? I assume because it is from a scroll and not a codex (bound book), and that lines are scored (sirtut); a law required for Torah scrolls (and tephillin and mezuzot) but not regular books.
Daily Hebrew posts a wonderful visual of what this picture (artfully cropped by Artdaily.com) looks like:

1 Vellish
2 Bet Yosef
3 Ari