This is a broad topic. Can you give us a little more information?
Do you want to know if your child is meeting your State's reading standards? Or do you want to know if he/she is reading at a sixth grade level?
State Standards are often measured by testing. For example, I am in Florida and we use the FCAT (Florida Comprehensive Assessment Test). The reading test is given to 4th, 8th, and 10th graders. Results show strengths and weaknesses based on the FL standards. Now, keep in mind that these scores may not be accurate as some children do not test well.
If you want to know if your child is reading at grade level, you can go to the library and find books that actually have the reading level printed right on the back. The librarian can help you. The number could be 6.4 - which means 6th grade, 4th month. Have your child read silently, then ask them questions to check comprehension. You can also ask them to read out loud to check for fluency. Have them find a part they would like to read aloud. Allow them to practice first. As you listen, you can determine comprehension often by his/her inflection. If the character is angry, your child's voice will reflect that feeling. You can have them do a cold read (mean they haven't seen the passage before) to check for word recognition. All of these points will give you information on how well your child is reading.
You can also contact your school reading coach and ask them to test your child. It will probably be an informal assessment, but these assessments can shed some light on your child's reading ability.