However I can certainly find LHR-ATH-FCO (oneways or returns) available to book on Expedia. On the A3 website, you may need to use multi-city function, and even so they may not be bookable.
And to answer the question in the title, searching on ITA will give you a fare basis and rules even if that ends up not being bookable - and entering random dates for FCO to LHR route A3+ gives various fares from ROM to LON which have rules such as NO DIRECT FLIGHTS ALLOWED and STOPOVER IN ATHENS PERMITTED AT EUR 75.00, indicating that this is one of the fares the OP is looking for, not just a combination of two fares.
If booking through Expedia (or other OTAs), how can you tell if what you bought is a one-way, or a combination of two one-ways? Usually you cannot see the fare basis and rules before paying, but is there a reliable way to always get them after the flights have been ticketed? Is looking up the PNR and seeing a single eTicket number enough to guarantee that it is a single, one-way ticket?
Many of the European AAA-ATH-BBB combinations have very short connections in ATH, so if the inbound is late and you're on two separate tickets, then you'll likely be out of luck and forced to cough up the money to buy a new ticket yourself. Or will the OTA always pick up the bill in this case?