Although Texas homeschoolers are NOT required to take standardized tests (no, not even the TAKS), most homeschoolers choose to administer some type of nationally-normed achievement test yearly. Third grade is when most homeschoolers begin letting their children take this type of test. Some choose to administer tests every two years. Others administer a test at the beginning of the year, and at the end of the year. It is really up to you. With that in mind, I've put together some web sources that could help.
Sonlight provides free diagnostic tests so you can determine at what level you should start your child. I suggest testing them at a level below what you think they should be on.
Mindsprinting- Take free reading and math assessments here. The site provides tutoring sheets to help teach the concepts that your child need help with. The price for tutoring sheets is around $50 per year.
Below are some tests to help you in determining your child's learning level.
K12 has printable Math and Language arts tests
The Oklohoma Homeschool contains a list of the various assessments available. It also has great info on record keeping.
PHILOSOPHY BEHIND TESTING
Before testing, you should understand the arguments for and against standardized testing. Here are a few articles to get you started.
Pros and Cons in a Nutshell by John Messerli
College Admissions for Homeschooled Students by Greene and Greene