In the many hundreds of PCs I've seen, and the many I've built for myself and others, I have not once seen an external label for the PSU.
Me neither.
Then they have just a 4 year life span. First PAT test they fail unless tester is willing to dismantle them!
Are you talking about a label?
I would say that would be pretty superfluous. All desktop/tower PC PSUs (ie. not laptop) that I have ever seen have been Class I. Any PC frame they are fitted into will also therefore be some flavour of Class I.
To mess about looking for a SI transformer label, and failing the appliance because you can't find one appears to me to be a massive, pointless waste of time. Give the whole assembly an appropriate Class I test sequence, and move on.