Hi Ramón, the stability of a compound in DMSO or in other solvent solutions depends strongly from the special characteristics of the compound. In our experience, there are compounds which are very stable and resistent to multiple freeze-thawing cycles in DMSO. Other compounds are very sensitive, like Calcein-AM from Life Technologies, which we only use as fresh dissolved solutions. In this case the characteristic of DMSO to easily take up moisture is a big problem. To achieve a maximum of stability of your DMSO solution you should also use a compound concentration which is as high as possible (e.g. 10 mM), this is also true for all kind of compound solutions. Since you don't know whether your compound enter the cell, I would recommend to use only freshly prepared solutions in your experiment.

Hola Ramón, 

The stabolity problem should be by oxidation or hydrolysis. 

To avoid oxidation you can buble your DMSO stock solution with Nitrogen, to eliminate the air and store in vials that close really well, using also parafilm arounf the cap. 

To know if your compound is working, first you have to assay your strains with fresh prepared solution. Then you can start using your stock solution and check if after a cycle of freeze-thawing keep providing the same result. 

Additionally, you cam check if it is stable running  a TLC or  HPLC or GC( depending the polarity and the MW of the compound)