how do i know a substance is an acid or base without litmus paper
In the olden days, you could taste it, can't advise that anymore. Some acids you can smell (acetic, hydrochloric, nitric, sulfuric). You can drop a small piece of chalk in it, if acid, it bubbles. Some bases smell also, ammonia is a common one at my house.
So the answer to your question: You have to have experience on what to detect, and how to do that, and be careful. This is why I always taught the final essential step in diluting chemicals: Label that which you prepared immediately, not putting if off.