You can know your CNA number by going to the CNA registry page for your respective state on cnatips.com. Click on your respective state and then click on the nurse aide registry page on the right hand side of the page to go to the homepage of your state's nurse aide registry.
Go to the professional licensing agency website for your state and there should be a search there. I've looked up my cosmetology license for Indiana before. You can look up all state issued professional licenses.