I honestly did try to find a dedicated topic/thread, but failed. Apologies if this is misplaced.
As many have noticed, over the last years many airlines and OTAs have hidden or entirely removed the opportunity to see specifc fare bassis codes (like J, Y, W, etc) while booking a flight. This of course is a big problem when one wants to be sure about miles accrual for the future flight. The questions is, do we know any OTAs which would make the codes visible at the booking stage? I used to use Expedia, but it seems to have changed there as well.

Checking out matrix.itasoftware.com is a great place to check out fares and see what the specific booking code is in.

Need to book other places, but a good start.

ITA Software Matrix Airfare Search Consolidated Information and Help Thread more info here.

Thank you, I have been occasionally using this for years to varying success, as it is increasingly common that agents are reluctant or unable to book the fares indicated by Matrix. So I was hoping someone knows a solution which doesn't involve two separate tools...

You are asking for too much! Use ITA Matrix which will almost always tell you the correct booking class you are looking at with an OTA

Well, just a year ago Expedia was doing exactly what I am after, so I don't think that this is exactly asking "too much".

If you're booking directly with the airline, there can be airline-specific ways to see the fare basis code, such as going to advanced search if available or starting to purchase the ticket, then clicking to see the fare rules (and therefore the entire fare code, which often can telll you the fare class as well as other information about the fare if you know how to decide it for that airline.

Another idea is to check fare class availability on EF. If a fare class isn't available (for instance, when searching on EF, you see I0) there's a good chance that your ticket won't be that fare class (I in this example), although married segment availability can mess this up. Also, the fare class can be available, but there might not be any applicable fares due to restrictions such as advanced purchase or minimum stay requirements, although this is likely to lead to a higher fare class, which is better for earnings.

It appears to me the airlines (that i'm dealing with at least) are going out of their way to hide the booking classes so you never know if you 1. accrue miles and 2. build any possible loyalty status until it's too late without havig to do a lot of legwork to find out this booking class information in the first place.

I just had TAP (who I status matched with with Oneworld Emerald AA) tell me my SWISS airlines longhaul expensive return flight segment accrued zero miles, segment or miles status towards TAP because it was booked as a 'K' booking class. There was zero information on this anywhere when booking and on any emails received from Lufthansa (or SWISS airlines) who issued the ticket. The outbound segment on Lufthansa actually earned miles.

So based on reading this thread I need to use the 'ITA matrix' to find out what the booking class is? Sorry for my ignorance but will this matrix cover for a flight on a partner airline issued on another partner's ticket stock?

Can it not be a reasonable request for all ticket issuers to show the booking class of segments so they can make an informed choice? Is that really too much to ask?

MIAman3 apparently it is

Forget ITA Matrix, since ITA only shows GDS prices, more and more airlines have filed NDC fares now which are cheaper than EDIFACT GDS, so you won't find them on ITA. Some OTAs have already NDC content available, and if you book directly with the airline, then of course its the same price as NDC content.

But to come back to your question:
Unless you find an OTA that can show you the full fare rules, which most of the time include the basefare on top of it, and in 99% the time the first letter of the base fare is the booking class, you won't know which class you're getting.

I'd recommend then to start booking through an actual travel agent, then you get the best of both worlds, lowest prices (+service fee) and full service.
When looking for one make sure you ask them if they already have access to all NDC fares with all airlines that support NDC already.