Thanks R&B for the tips.  I believe my SS pearls have a good lustre and are evenly coloured.  All of mine were already set in pendants, but have small caps apart from one.  
My round (as far as I can see) pearl has a large domed cap (acorn-like) covering part of the top of the pearl, but the pearl looks fine otherwise.  I suspect the pearl might have had a circle around the top or been a bit pointy, I will never know.  But it seems too extreme to get all my pearls valued/x-rayed before buying them, it would really add to the cost!
Once however, while out pearl-hunting I saw a two-tone SSP in a pearl jewellery shop.  What could this mean?  I think it was a drop shape, and the top was cream and the bottom was light gold, or vice-versa.  Is that bad?  I didn't go for it.  Sales staff were very pushy.
Also, have been researching x-ray pics and found this very helpful photo and description from this website: 
http://www.michaelfreemanphoto.com/media/429853c2-272d-11e1-be7f-0d815625950d-pearls