Technical Answer: You should be able to use the setError method on your radio buttons.
Here is the setError on a textbox:
UX Answer: It is preferable to have a default option selected in a group of radio buttons. There are some instances not to have a default, but really think about what you are asking, and consider if a default or another set of controls would be more appropriate.
See this excerpt from MSDN:
Because a group of radio buttons represents a set of mutually
exclusive choices, always have one radio button selected by default.
Select the safest (to prevent loss of data or system access) and most
secure and private option. If safety and security aren't factors,
select the most likely or convenient option.
Exceptions: Don't have a default selection if:
There is no acceptable default option for safety, security, or legal reasons and therefore the user must make an explicit choice.
If the user doesn't make a selection, display an error message to
The user interface (UI) must reflect the current state and the option hasn't been set yet. A default value would incorrectly imply
that the user doesn't need to make a selection.
- The goal is to collect unbiased data. Default values would bias data collection.
- The group of radio buttons represents a property in a mixed state, which happens when displaying a property for multiple objects that
don't have the same setting. Don't display an error message in
this case since each object has a valid state.
Make the first option the default option, since users often expect
that—unless that order isn't logical. To do this, you might need to
change the option labels.
Also, the button should be on the left, with the text on the right.