Skill checks do not have any markings on them by default, but you can choose to add tags for lines in the dialogue system, i.e. "Persuade", "Poison", "Herbalism". Any line can be checked by any condition (plot variable, script), doesn't matter if it's a player selected line or NPC line.
The way that dialogue is usually structured in Thirst is that a line will appear regardless of skills or attributes, but the follow-up line will be different depending on a skill check. So if I pick a persuade line, I have two possible outcomes: fail or succeed (or perhaps something more nuanced).
However, dialogue options can also be checked for conditions too, which is how many quests are handled (i.e. "I've collected all the rat butts, give me my reward"). Some lines which I feel
should not show unless your character can do them will also appear selectively. Usually this is related to stuff like your current class (i.e. if mage -> line about magical stuff appears), or your attributes (most often cunning/intelligence). In these cases, usually selecting the option will only lead to one possible outcome, but there are still some situations where there are multiple nested dialogue checks.
The Dragon Age dialogue system makes certain things easier for you. You can mark lines to appear once per conversation or once ever, so things like quest-relevant options can't be selected more than once to obtain multiple rewards, or you can "check off" dialogue options so the player won't keep being presented the same options over and over until the conversation is initiated again.
This also lets you get away with a lot of stuff like not needing a unique variable to update when you talk to a minor NPC who only has a few conversation options (usually a "first greeting" that only appears once, and an "oh, you've returned" line). You can also link the same lines to appear in multiple places, which is an extreme time-saver and organizational aid, but you have to be careful you don't link a line in the wrong place (for instance, maybe you have a line that appears 3 times, but with different conditions each time... you have to make alternate versions of each line, or do the check on a previous line before the link).
More complex dialogue requires tracking of several if not dozens of variables through conversation and smart dialogue tree structuring, especially when you might have multiple quests interacting with the same NPC in convoluted ways. Some NPCs in Thirst have 10+ conversation trees because their dialogue has to vary correctly, show certain options or deny them depending on various quests that may or may not have been completed, etc.
