What was the first evidence that revealed to uri and stopthief that the nazi soldiers had begun their persecution of jews?

The two see evidence throughout the first eight chapters, but Stopthief doesn't really understand that it's persecution of "Jews," until around the 8th Chapter. The Nazi invasion is evident everywhere; tanks are present; bombings regular, general disorder and panic is the norm.

Uri knows about the Jewish persecution from the beginning, although he may not understand it. The yellow stone around Stopthief's neck and the fact that he recognizes it is enough to tell us that. Uri decides that Stopthief is a gypsy, but the reader isn't sure yet. All we know is that Uri and his friends joke about the Jews.

Besides the signs of Nazi artillery, persecution can be seen in Chapter 6 when they witness the man cleaning the sidewalk with his beard; in Chapter 8 they see the vandalism of businesses and confiscation of property. If you look at the timeline, you can see that the evidence appears a little at a time. It's like a puzzle revealing itself one piece at a time.