Yeast viability after a month has decreased quite a bit, depending on the type of yeast you have. Wheat beer yeasts degrade rapidly where something like a Cal ale yeast lasts longer. If you don't smell any autolysis (burnt rubber) or off flavors it should be OK. If you wash the yeast, make a starter and repitch it should be fine. It also depends what type of beer the cake was from. If it's a high gravity beer the yeast cake may be too stressed.