Foods sold under the Cottage Food Law must be non-potentially hazardous.
The definition of a non-potentially hazardous food is on pages 11-13 of the Texas Food Establishment Rules.
There is a chart that you can read to determine whether the combination of pH and water activity (aW) makes that item potentially hazardous or not.
If you have a food that you would like to have tested, there is a company in Texas called Food Safety Net Services
which will test a sample of your food for pH and aW for around $36.00, plus a setup fee of $100. (Setup fee waived if lab services total more than $1000, as of Feb. 2014). They have offices in Dallas and San Antonio, and you can either bring in your sample or overnight it on ice.
This is a great resource for Texas home bakers who are wondering if their recipe is allowed under the law.