PST is for SEALs/EOD - IST is the 1.5-mile run and such. You will probably take an IST with your recruiter, and then one in Receiving at boot camp. You will NOT take it at MEPS.
"bend over and spread 'em." Good luck and you will do an IST the 2nd or 3rd day after you pick up with your platoon in boot, and you will have to do one within 30 days of your ship date.