Do you know any sources regarding how the Jews determined Rosh Chodesh during the times of the first galus, between the two Batei Mikdash? It’s hard to imagine that it was through the typical process of testimony, and the conventional setup we use now only seems to be mentioned regarding this latter galus, post Bayis Sheini. What did they do back then?
I’m sorry, but I don’t know much about this.
Why do you assume that this was not done by means of testimony?