Yes, they are both used for emphasis. The difference is the same difference that a period and a question mark usually have: In the first example, "...., you know." a period (indicating indicative) means that the speaker is stating or telling the other person that there is no excuse. The speaker is providing information to his listener. In the second, "..., you know?" a question mark (indicating the interrogative) means that the speaker is asking the other person if he or she knows that there is no excuse. But this is could be used as a rhetorical question, indicating that the speaker does not really want an answer. In this sense, he is also providing the other person information. So, yeah, to be short, they are both used for emphasis, but in slightly different ways.

I see you are in the UK, I am only fluent in the American flavor of English! This is a good question, and easy to answer for this particular sentence structure. This is informal conversational English, probably won't be seen in a book except as a quotation. I think your first example is missing the '?'. At the end of a sentence, I would say that "you know?" is an abbreviation of "do you know what I mean?". So it is a question and could also be written several ways with the same meaning: - There is no excuse, do you know what I mean? - There is no excuse, know what I mean? [very common, even more informal] - There is no excuse, right? - There is no excuse, do you agree? I would say the speaker is mainly asking for confirmation that the other person is still listening. 'You know" at the beginning or middle of the sentence can be different. - You know, it is 10 o'clock and I am ready for bed. - It is getting late, you know, you should go home. I think this usage is just conversation noise to fill thinking time, but you know, most people use it!