Among the taller Cascade peaks, it's pretty easy to tell which ones have been volcanically active since the Pleistocene glaciation: they're deeply eroded and ragged looking. Three Fingered Jack, above, is an example of this. The central lava conduit(s) are generally more resistant to weathering and erosion, but during their emplacement, as they exsolve various noxious gases and water, and heat surrounding groundwater, they cause hydrothermal alteration in the rock around them. This renders the country rock weaker, often just a semi-coherent mass of clays and similar minerals. Had there been post-glacial eruptions here, the deeply eroded altered areas would have been buried below some combination of lava and tephra (a loose term for fragmental volcanic products, including ash, lapilli, and bombs)- in other words, the edifice would have been smoothed out to a "more typical" conical form.
Several peaks in this area, including this one, Mt. Jefferson, Mt. Washington, and North Sister (largely cloud enshrouded in yesterday's panorama) have similar forms, and have been inactive for the last ten thousand years or so, and are unlikely to erupt again. This should not be taken as "proof" they're extinct, but smart money wouldn't be betting on them to erupt.
This photo was taken before yesterday's, lower on the trail, before we really started the climb around the cinder cone to the north of Sand Mountain, so the location is approximate.