Both. Website at the bottom of the shopping cart on the right side, anytime after you choose a flight. Mobile app anytime after you choose a set of flights, click the arrow to expand total due details at the bottom, and cart ID is just under the continue button.

Cart ID shows the agents in their reservation platform the exact flights and booking codes for each flight you were trying to book, including the full fare construction, in addition if it is valid for them to honor or not. The difference is you are now dealing with a lot of agents who aren't familiar with how to manually store a fare from a construction or how to change the booking code to match if it's not currently available. The problem is a lot of these agents are also taught to only book origin-to-destination, and not individual segments, which was often the work around web support agents used to honor some fares. And when you're also lumping these calls back in with a group that is primarily judged on how many calls you get through per hour, the answer is often take the worse customer experience and say it "can't be done" to get them off the phone. If they're smart, they would be looking for trends in which cart IDs generate the most issues to decide where to prioritize their fixes, but who knows what they're actually thinking/doing some days. Probably a disconnect at the leadership level who made the decision between agents having the tools vs. knowing how to use them. Sure, every reservation agent may have the same tool, but you're definitely dropping these calls into people who don't know how to use the tools to solve these types of issues.