Say you have a fairly simple recipe that you like. A base malt, one specialty malt, one hop variety, and a readily available, consistent yeast. Simple enough that you can make it consistently.
Now, vary only the mash temperature. Mash the first batch at 142 °F. Mash the next at 150 °F. Mash the last batch at 158 °F.
Do a side-by-side comparison and record your observations.