Is this a specific PC, or a general question?

All spyware needs to get the info to the spy somehow, so as others have said, it's probably best to watch the network traffic.

But also - not so much for home, more for an office - look out for keyloggers and weird physical devices attached to the PC, they can be sending data via other methods.

And prevention is better than cure, get a good antimalware installed - and perhaps something which only allows known good (allow listing), rather than blocking known bad (block listing).