Go through with Annexin V staining based assays. Other wise if you want to check level of DNA fragmentation, use TUNEL assay

You could also do cleaved caspase 3 IF or Western. If you are looking for something more fancy Cell Signalling and R&D systems have different apoptosis assays, such as apoptosis arrays and mitochondrial membrane potential assays to complement the TUNEL, Annexin V and caspase experiments.

http://www.rndsystems.com/product_detail_objectname_apoptosis_detection_kits.aspx

A little bit more information would help me get my head around what you wish to do. Are your cells adherent? Are you wanting to look at levels of spontaneous apoptosis? Or are you looking at sensitivity to induced apoptosis?

Analysing levels of death in culture is not necessarily simple as inducers of death that take some time can complicate matters as levels and rates of cell division may also need to be included.