Do you know how to look at a chicken's butt to see if she's laying? Turn one upside down and look at the vent. Either it looks small and puckered and rather dry (not laying), or it looks larger, softer/wetter, and stretchy (because an egg stretches it each day.) If helps to look at some that you know are laying and some that you know are not laying, to see the differences. A rooster makes a fine example of not-laying, and a chick would also be an example of not-laying.
If you're getting eggs you think might be from her, you could try putting her in a pen for a few days, with a nestbox (and possibly with another hen for company: pick one who lays different-color eggs). You would soon know if she is laying, or if the eggs in question are being laid by some other chicken.