I would recommend carrying out a time course following induction of HO by galactose and monitor the generation of the DSB using a Southern blot.  Take an aliquot of cells prior to the addition of galactose and then take an aliquot at 30 minutes, 1 hour, 2 hour, etc.  If your induction is successful you should detect cutting within 30 minutes and certainly by 1 hour.

Thanks Glenn!

Is there another way to assess HO induction levels? I was told that it is possible to count yeast colonies before and after galactose induction. Because the strain I am using can not perform homologous recombination (hml and hmr deletion) after proper induction there should only few survives.

Plating efficiency is an indirect way to suggest the induction has occurred.  If you want that is sufficient for your analysis then that is an easy approach.  If you want to have a more rigorous assessment of DSB formation then the Southern would be a better approach.

So every time You do an experiment with HO endonuclease you take samples for Southern?

DO you grow yeast on raffonose or lactate before galactose induction?

We typically grow our cultures in 3% glycerol lactate media prior to inducing with galactose.  Good luck.