For me there wasnt really a need to ask. Figuring it out was easy when the driver did not speak English and/or presented the standard fake mexican drivers license. Then, Border Patrol was called, they took over and took them away. I didnt have to ask them anything about their nationality. However, a squaddie of mine whose jurisdiction is in the NE, does not have access to Border Patrol officers to the extent that they are available in the SW. Without BP available, the only option is to let them go whether you know they are illegal or not.