I highly doubt she'll volunteer information about OM, so is this something that I should ask about?
You NEVER "ask" about an OM/OW. You only STATE ("I know all about you and ______,"), or you just assume it's going on, or you respond to HER if/when she asks "What will it take for me to return to the marriage?" ASKING is supplication, which is weak and unattractive, plus she will only lie anyway.