Perma-Pier

and there's your sign, that is ground movement, if the cracks did not expand during the drought then you should be fine on the foundation. expansive clay is predominant in this part of the country and will make a house shift and move yearly. there is not much you can do about it except the soaker hose. one of the problems with the way slabs are done around this part of the world is they are usually not monolithic and have sand placed inside the stem walls for fill. rock (3/4"-1 1/2") is a much much better backfill but is almost never used.

as far as the mortar joint method, that will work good if the brick was laid level. there will be deviation in the mortar lines no matter how good the mason. better way is to find 4 or 5 spots in the area you think is moving and mark the concrete about 6" above dirtline. measure up to a brick course (15th course as an example) and write the measurement down for each spot. remeasure every 2 weeks for 3 months or so and see if they match within 1/8", if so then your good, if not then up to a 1/4" movement is acceptable. more than 1/4" and its time to find an engineer.

Hey StumpJumper, I'm in Rockwall at least 3 days a week, If you want me to swing by, let me know. I inspect about 600 houses a year.