What is one way to test whether an unknown solution is acidic or basic?

Answer: There are several ways. The first that comes to mind is a pH meter. A pH electrode Is lowered into the solution, and (Assuming) the pH Meter has been properly calibrated, and the temperature of the solution is set to the calibration of the Meter, the pH can be read directly from an analogue scale or digital readout. Below 7 is acidic, 7 is Neutral, (like Pure Water), and over 7 is Alkaline, or Basic.

A useful, but less accurate method is the use of any number of "pH Indicator Solutions", which are essentially a type of various colored dyes that change color within differing pH ranges. Usually, if the pH is unknown, a small amount of solution is removed from the container and tested separately - in a "well plate", or similar method.

These types of dyes, or Indicator Solutions, can be dried upon strips of "pH indicator Paper", which, depending upon the type can be very useful when carrying out more precisely arrived at pH tests like Titration.

Just to see if a solution is "Acid" or "Base", Litmus paper is used; "a Red color shows Acidity, and a Blue color, a Base"; ergo, "An Acid Solution will turn Litmus Paper, Red".