Okay.
If the problem is in DPS, it's readily apparent by the fact you hit enrage.
You can tell by certain skill animations being seen or not. I raid with MNK/MCH/SMN. I know what I'm supposed to see on Faust with those three, at least. Generally I'll give it one run where I don't bother paying attention to others. If there's a problem, I'll watch. A bad MNK will sit on one side for far too long; they're clearly missing positionals or even skipping buffs entirely. You can see most of their main ones just in the party list (TwS and FoF) and the dots on the boss itself. SMN and SCH are easy to notice, because there should be 2 instances of each debuff up at all times on the boss. If there aren't, one or both aren't doing their job proper. BLM you have to actually watch the party list, so it's a little more difficult; gotta see Enochian not falling off, see them cycling fire then ice, etc. The rotation is complex, but simple. If Enochian drops off against Faust, they're clearly doing things wrong. It's a dummy. Bards are EASY to tell just by enmity list, as are MCH. Both of them SHOULD crap out INSANE enmity on the opener, even with Quelling Strikes activated - it's why they should be using it.2) How do you plan to prove someone (or two of them) is/are the problem? You mentioned that "you can tell". Exactly how? Enmity bar? The only job out there without an enmity reducer would be monk. Every other DPS job has something that affects enmity. The enmity bar is also rather limited, especially if the main tank's going all in on enmity generation, making other player's enmity bar as large as 5 pixels.*
These are all general "feelings" if you will, but the overall point I was making was more about raid dps and less about individual classes. It's really easy to notice if a boss is dying considerably slowly, when I've done the content to death. I know my rotation. If I'm going for too long, I can tell.
That's an issue with the game's design, yes. There's no positive feedback loop for performing well with DPS classes. Yeah, as Machinist, you'll see that HUGE hit on Wildfire or Bard will see that big Empyreal Arrow. Hitting 5k crits on Full Thrust as Dragoon feels SO GOOD. But none of that translates ~at all~ to how you're performing. Everyone can make big hits happen. Very few people can have that carry through an encounter.3) How can other players on DPS roles know they're performing well or not without a parser?
I think the game should have personal parsers for Striking Dummies. I definitely definitely think so. In instances, though? If it's anything other than a PERSONAL meter to show YOU how YOU performed in the encounter, I disagree with it. Completely. It's nobody else's business how someone else is doing. It's not your job to tell someone they suck, it's their job to seek help to get better.*
99% of things in most guides to playing classes can be reasoned out and calculated mathematically given the numbers provided by the game. Optimal rotations can be calculated off potency amounts. Simple testing shows us that potencies are essentially percentages. Your auto attack is ~100 potency if you compare it against your other skills and their potencies. Due to this, you can easily assume that potency directly correlates to damage output. If you produce a rotation that spits out maximum potency per gcd, you've similarly discovered a rotation that spits out maximum damage per gcd.4) If said players are supposed to find a guide to "perform well", what makes that guide credible if no parser is involved during the creation of it?
You don't need a parser to do that math. The only time a parser is really employed is when discerning best in slot sets, since stat weights require rigorous testing to discover damage formulae - and even then the parser is just there to make number gathering a bit easier, I feel. You can similarly just record it and add up all the damage ticks, divide by the time and boom done. It's just more time consuming that way.
I personally created this guide for playing Dragoon a while back without employing a parser in any part of it; it's a result of my own trial and error in playing the class, alongside calculations I made like those I described above (and then a few tips pointed out to me by others that I hadn't discovered yet). Would you say my guide isn't credible?
*//EDIT: That is to say: Once a feedback loop exists where players can see their own personal damage/dps scores in an instance, it's possible to then DISCUSS it with other players. Talking to other players of your class to be like "Hey, I did 700 dps in Alex 1 today. How are you doing?" if the person responds with a higher number, the lower number person can either attribute it to gear or them lacking in certain skills. It opens dialogues about the classes and how to play them in-game without use of third-party programs. THAT is a change I would like to see. I would never want to see it where people start laughing at the poor newbie NIN who is struggling to maintain 600 dps in standstill because of the complicated dance that is their rotation. Once they see that they're doing worse than others of the same class, they can seek to improve. Being TOLD they suck isn't gonna help. It'll just make them feel worse. x: