Hello and Welcome to justanswer Renault..
You can usually tell to a great extent by the price you paid - ie, a "new" engine as you mention costs many hundredes -( I cant be precise but any Renault dealer parts dept with a phone call will give you the new price)
Engines that are supplied from engine supplied from engine sellers these days are usually "reconditioned" -unless specified "new" or other.. you would need to raise the issue with the supplier.
I suppose its just possible that you have been sent an engine that was in their stock - cleaned and ready for rebuilding / reconditioning - but not actually done as yet - as a complete msitake - Thats a thought - I could tell if I saw it - ask your mechanic if he thinks thats a possibilty / thats happened... in which case, they would have to replace it for you.
It is an odd situation - the fact that you mention "old oil" in the sump made me think of the above -
Im afraid there is no way you can check with engine numbers - there are no records kept of engines that have changed hands like that - only complete vehilce Vins when a car is scrapped.
So the best way forward is to contact the sellers/ suppliers - tell them exactly what you have found - ie, the old oil in the sump and the carbon in the head - and see what they have to say - - - and then go from there...
I Sincerely XXXX XXXX helps you... let me know if I can help anymore...
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