the one that has the most stress is probably going to depend on if the trailer is riding a little nose up or down. mine tends to a little up. looking at the axle when hitched, the front axle appears to have more bow in it. i believe the front (on mine) is sliding more when turning. the back is planted.
i would feel the frame and see if u can feel any raised area. that brace will tend to bend it out a little if it is stressed. i've seen pictures on other sites where the crack was at the 12 o'clock position. the ones pictured in the above are a couple of inches from the lower part of the "I" on the frame. it all depends on where they put brace on the inside. from the underside, u can push up on the underbelly cover and find them.
my cracks are related to the rear axle. i can feel the bumps on the front but so far, no cracks.
i believe lippert recommends one between each hanger; in my case 3.