Detemine if SED is successful

Is there a way of finding out if an issued sed command is successful?

Basically, in my script I have a command where mydata=$(cat record | sed '...command here...'). When the sed command finds the data that I am after it is assigned to mydata. When the sed command does not find the data I am after, xxxx is assigned the whole contents of the file, which is expected. I am processing a file using a while loop. I was wondering if an indicator is set to indication success or failure.

Unless an actual error occurs sed returns an exit value of 0, so testing the exit value is not an option.