Most yeast strains list a value of apparent attenuation (usually 75-85%). Attenuation = the amount of available carbon source the yeast will consume before it begins to flocculate and go dormant. This is an easy way to determine if you fermentation is complete. Say you had an OG of 1.070 and a yeast strain with apparent attenuation of 75-85%. Your fermentation would be complete when your FG stabilized with in the range of 1.0105 1.0175 any value in this range wit a 2-3 day consistent reading indicates that your fermentation is complete. Higher values indicate that you have a stuck fermentation, which could be due to unhealthy yeast, lack of nutrients, under-aeration ect
Some bacteria and wild type yeast are consider super-attenuators so lower FG values may be indicative of an infection. Hope this helps----
Jason