its difficult as most of time we do not know when they verified (or verified or not) and they do not concerned with present of candidiate at the time of verification. they are concerned with real data and duties mentioned by employee is true or not.
again like me my letter and all details are form chennai HQ (all ph no and email name of superier etc) and I am sitting at ahmedabad now if they check at chennai anyhow i will never be available there (may be I be there at routine visit there which is uncertain)
and i dont know the HRD like us having more than 2000 employee can assure NDVO directly by name that i am working there or not. but yes on my letter they have letter number by which they can track and confirm them.
about previous employer again its difficult to know that do they check there or not
I think we should also track the verification date done by NDVO on employment part by phone or personal visit. I have seen cases in past where the candidiate was not present or was on holiday & NDVO rejected their case. We must track it so that we may come to know about their visit or guess on it.