thanks for the heads up. Since we were thinking of going to Montana this summer, I did a quick internet search and came up with Montana's laws on invasive species. It is listed at http://fwp.mt.gov/fishing/guide/ais/lawsRegulations.html
and further info on the specific law at http://www.leg.mt.gov/bills/mca/80/7/80-7-1011.htm
In part it states, "The departments shall establish a check station within or adjacent to an invasive species management area to prevent ...."
Looks like there should be inspections stations near popular launch sites. I hope I don't have to search endlessly for one.