Invertase is a generic term for any enzyme that hydrolyszes sucrose into its constituent monosaccharides glucose and fructose. There are two basic types: alpha-glucopyranosidases and beta-fructofuranosidases. The net effect of either is the same, i.e., release of one fructose and one glucose from each sucrose molecule. These can be analyzed spectrophotometrically by any reducing sugar method, such as the Somogyi-Nelson arsenomolybdate procedure, the bicinchoninic acid technique, the dinitrosalicylic acid method, etc. Just bear in mind that one unit of invertase will release two micromoles of reducing sugar per minute. Another technique is the measurement of D-glucose only, by an enzymic method such as glucose oxidase or hexokinase. If you want to know if you have an alpha-glucosidase or a beta-fructofuranosidase, you can use alternate substrates such as pNP-glucose, raffinose, etc.