How was pH measured back in the day if you had nothing to calibrate to?
&quot;Hydronium&quot; content can be easily determined by simple acid base titration. Keep in mind that there is nothing fundamental about pH. It is just a convenient scale for expressing hydrogen ion concentration. For example, a kilogram is a recent invention for expressing mass measurement, it does not mean that ancient people did not weigh anything before kilogram was known.
In the same way, acid-base titration is more than 300 years old, with litmus among the earliest indicators. pH is a relatively new concept. Of course, the titrations were not analytically precise and accurate 300 years ago.
Historically, the simplest way to do acid base titration was to use potash (naturally occurring mineral $\ce{K2CO3}$). One could continue titration until the evolution of bubbles stopped.* No indicator needed. This is the way vinegars were assessed. Note that in those time moles, molarity, did not exist. There was the concept of equivalent weights, i.e., this weight of X will react this that weight of Y completely. The concept of equivalents is still taught in some countries, although it has vanished from Western curricula.
Kolthoff (father of modern analysis) has written books on acid base indicators. Check internet archive. Your question is worth an entire essay.
*From Bishop, Indicators: International Series of Monographs in Analytical Chemistry