Unfortunately, there isn’t any indicator of what expansion a quest is for. In most cases, it will be fairly obvious because a given zone will be from a specific expansion. Things only get confusing in areas that are mixed timeline. Major Cities are one example, but you also have some zones with different phases, so which expansion it’s in depends on the phase it’s set to. By default they’ll usually be set to the most recent expansion’s phase. In these zones, you’ll see a chat bubble icon on the map marking an NPC that can change your phase.
Chromie Time mostly sets older expansions so that they scale all the way to 50 and also will determine which dungeons you see in the group finder. From your post in another thread, it looks like you haven’t unlocked Chromie Time yet, so you won’t have access to those features.
That doesn’t stop you from doing quests from other expansions, but you won’t be able to turn on scaling or use the group finder outside of BfA dungeons. If you want to run Ragefire Chasm you’ll either need to find a group manually or outlevel it enough to solo it. It’s a bit of a weird case. While in general, “Vanilla” content will start giving you a substantial overlevel bonus at level 35, for some reason, RFC uses Mists of Pandaria scaling (I assume it’s a bug due to the RFC map being used for other MoP content. I reported it long ago, but checked today and it still seems to be using MoP scaling.) So, for MoP scaling, you’ll start getting a modest overlevel bonus at 40. (Enough to solo RFC.)
If you want to level outside of BfA, you’re main option is going to be Legion, as it will scale to 45. While you’ll start seeing an XP penalty after that, you should still get enough XP to get high enough for Shadowlands. To start Legion, go to a Warchief’s Command Board, and select the “Broken Shore” option. (If you don’t see it, pick up some of the other quests on the board to unbury it and then abandon them once you’ve picked up the Legion one. Legion is usually one of the top level options, though.)
If you aren’t in a hurry to level and just want to experience some of the other expansions, let us know which one and we can point you towards the proper starter quests so you don’t have to trial and error all of the quests available in the major cities. If you go that route, you’ll need to decide if you want to lock your level to keep things challenging, or if you’re okay one shotting stuff. (The second one has the advantage that you’ll be able to solo group content, but some people get bored one shotting stuff.) If you want to go the XP off route, you’ll need to lock by 34 for Vanilla, BC, and Wrath content, 39 for high level Cataclysm (Vashj’ir, Hyjal, Uldum, Deepholm, Twilight Highlands) and Mists of Pandaria. 44 for Warlords of Draenor, and 49 for Legion. (Although the bonus for Legion is pretty modest at 50, so while stuff will die quickly, it won’t necessarily be one-shot territory so I wouldn’t bother turning off XP for Legion.)