Gel electrophoresis might help. Please check..but it would be for isolation and then discrimination. 

Thanks a lot for reply, Radul.

However, I wonder how can we know exactly the ratio between dsDNA and ssDNA in the same sample? I'm a newbie in this field so please explain clearer.

Thank you so much!

And what is your sample? DNA alreadyisolated from the cells?

Hi Stanislav,

I extracted Staphylococcus DNA from QIAGEN Kit. For the first time, I want to check by PicoGreen or Qubit ssDNA kit. But using Picogreen seems not quite accurately and Qubit ssDNA kit has many bad review for ssDNA detection. So I need to find out another method.

Thanks for replying my question.

Sorry Tuong for the late reply. I was away.

even I just guessed it..Don't know whether it really works or not.

Perhaps this article will help:  http://www.nature.com/nprot/journal/v1/n6/full/nprot.2006.477.html

Please check..and let me know if you already found some answer..