How do you tell if something was once alive?

If it's currently dead. Sorry for the sarcastic answer, I actually don't know. That's the most reasonable answer I could think of.

(edit) I looked it up, and here is what I found:

Once living things are objects that used to be alive, but now are not. For example, dried flowers, a dead insect, and a fossil would all be examples of once living objects. Even something like plastic could be considered once living, since plastic is made from petrochemicals that come from carbon life forms that died hundreds of thousands of years ago.