The Minnesota Gaming Control Board, which does have a website BTW, outlines payout percentages set for certain games of chance such as slots, keno, video poker, etc. I believe the minimum payout in Minnesota on a slot machine is 85%. Minnesota allowed Indian Gaming in 1988 and later added Class II machines. Although the MGCB does not do testing of the machines itself, the site does state that all slot machines are tested and audited by outside companies from Las Vegas and Atlantic City.
Additionally, with state-run gambling rumors and racino talk in Minnesota, the established Indian Casinos would have too much to lose by cheating their players out of the occasional win. I personally have had luck at all Indian casinos I've been to. Although one casino I won't mention does seem to take my money everytime (other friends of mine have done well there though).
If you feel as if your local NA casino is cheating you, don't go. Save for a road trip to another state or maybe just fly to Vegas once or twice a year. I have lost insane amounts of money in a short period of time before. OTOH, I have won insane amounts of money in a short period.