How do we know that one of them and not half of them are criminals

We've already done this, pay attention.

The research does not support the view that immigrants commit crime or are incarcerated at higher rates than native-born Americans.
...
What’s more, the arrival of record numbers of immigrants at the United States–Mexico border over the past two years has not corresponded with an overall increase in crime in so-called “blue” cities where many of the recent arrivals have settled. In most places, the opposite has happened — crime, including violent crime, has trended downward (other than larceny and a small increase in robbery) after peaking across the country in 2020. This has been true since the spring of 2022, the year Republican governors, including those in Arizona, Florida, and Texas, began transporting undocumented immigrants to cities with more immigrant-friendly policies, including Boston, Chicago, New York, and Washington.