It's a common practice to look for it on Google images, i.e. "Save Icon", and see if there's a common metaphor. If not, you look for similar use cases - something so that it's much quicker for users to grok what it does.
That's the fundamental purpose after all.
This was the problem with Google/Android's new icons as well as GitHub. Google got caught up with minimization and left out the telling differentiators. Githup went nuts on illustration and lost the original meanings.