Student or Learner
How can one identify a dense sentence in a text?
Welcome to Using English.
It depends on what you mean by "dense sentence."
To me, it suggests a sentence that tries to include too many ideas instead of breaking it up into smaller sentences. It could also be one that uses too many "big words" together, making it harder for the reader to make their way through. I find a lot of academic writing to be like that.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.