Lab technique to distinguish between single stranded and double stranded DNA?
Single and double stranded DNA have different electrophoretic mobilities, ssDNA will move faster on an agarose gel, so if you know the length of your DNA and it moves further than you expect 
 relative to a dsDNA ladder it's probably ssDNA. The hyperchromic effect means that ssDNA also absorbs more strongly than dsDNA, so you can heat up your DNA and if the absorbance changes it was dsDNA and if it doesnt then its ssDNA (you should take an aliquot for this so that you have the rest of your sample in tact). You could also take an aliquot and add a single stranded exonuclease and run the products on a gel together with the DNA before the digestion, if nothing is present after in the digested well then you had ssDNA if there is still a band after then you had dsDNA (i don't think this method is really used as you could just run it on a gel which is simpler and you lose less sample, but it should still work).