Diabetic is very complex disease Please go through the KEGG metabolic pathway- diabetic pathway and genes associated also it is associated with exercise sunlight(D3) etc. etc.

You could use certain cell types and check their response in relation to their function. i.e. HepG2 cells and their ability to handle glucose, Islet cell line concerning its ability to produce insulin while differentiated 3T3-L1 adipocytes are more commonly used where you check for glucose uptake by the cell.

As alluded by Puthupalayam, since type 2 diabetes is a syndrome and not a disease caused by a single factor, it would be virtually impossible to make valid conclusions on "antidiabetic" properties of poorly or undefined complex mixtures of compounds ex vivo.

If on the other hand, you anticipate a specific tendency of your extracts in that they may be interfering with for example, gluconeogenesis, or fatty acid synthesis, it may be possible to to generate useful preliminary data by using primary hepatocytes. But ultimately, you will need to use an animal model to better substantiate your contention.