The level of phospho ATM may be low in your cell line, so I would recommend that you use UV or another DNA damaging agent (IR, for example) that is known to induce ds DNA breaks. I don't know what wavelength of UV you plan to use, but a higher dose will ensure that you have a sufficient amount of damage to activate this signaling pathway. For example, we can detect increases in phospho-ATM when we UVB irradiate primary human skin cells with a minimal dose 90 mJ/cm2. Then run both control and cell stressed lysates and that will help you to determine if ATM is activated under baseline conditions. You can validate in a third way by purchasing cell lysates (such as Cell Signaling Technology) that will serve a positive control for pathway activation.