Actually, I am in a spanish cong in South Texas and the only legal immigrants in that congregation, aside from my family, are the elders and their family. This makes about an 30% legal, 70% not so legal. Many have simply overstayed their visas and the only reason the number of legals is increasing in my cong is they are having children. It used to be more like 15% legal, 85% illegal. Not saying we are the norm, but it does happen.
It was easier to judge before the assembly hall was built. You just had to compare and find out who did not make to Corpus Christi or Houston for the assembly.