It hunts. Matricula consular bank accounts, Illegal Alien home loan programs, trucks with shiny gold rims, etc.
That's only part of the equation, anyway. Enforce the laws against Illegals in the interior, and then there will be far less incentive to tresspass. Right now, 90% of all deportations take place when Illegals are apprehended within 72 hours of the border. Beyond that, the risk to them dwindles to zero.
Rounding up a few hundred thousand will create a stampede. The cost/benefit analysis is quite encouraging.
Simultaneously, assets of employers knowingly hiring Illegals should also be seized, and also auctioned to finance future interdiction.