NOT A TEACHER
Yes, there is a difference. By placing "you know" at the end of a sentence as in There's no excuse, you know. you are emphasizing the preceding statement (see also #18 know - Definition from Longman English Dictionary Online).
By placing "you know" at the beginning you are not always emphasising the statement as in the previous example. It can also mean:
You know (= you are aware that) there is no excuse.
However, it all depends on the way you say it. The intonation is very important because
You KNOW there is no excuse! (You are aware that there is no excuse!)
is different from:
You know....... there is no excuse.
- For Teachers