If the cephalexin is still packed in its original container there should be an actual expiry date somewhere on the packaging. Storage is also a factor to consider and we would only suggest using the product if it was in the original packaging and had been stored in a cool dry place (not the kitchen or bathroom) so under 25centigrade or 77 Fahrenheit is recommended.
If the medication has been repacked into a pharmacist bottle then I would say that it may not be usable. Cephalexin normally has an expiry of 3 years when it arrives in the pharmacy but there is no telling how long the pharmacy had it on its shelf. In addition having been repacked then it has been exposed to more humidity than the original container so degradation is a possibility.