One of the Critical strategy used to assess the antidiabetic potential of a plant extract without testing on mice is by studying the inhibitory potential of the extracts on carbohydrate hydrolyzing enzymes (pancreatic alpha-amylase and intestinal alpha glucosidase) and prooxidant (eg Sodium nitroprusside and FeSO4)  induced malondialdehyde production through a Thiobarbituric Acid Reactive Species Assay. Carbohydrate hydrolyzing enzymes are responsible for the digestion of carbohydrate with the end effect of hyperglycemia. The ability of our extracts to inhibit the activity of these enzymes is an indication of their antidiabetic prowess. Fe2+  has been reported to accumulate in the acinar cells of the pancreas which have deliterous effect on the beta-cells that produces insulin. These prooxidants can be used to induce lipid peroxidation and the ability of the extracts to inhibit these Knoevanogel condensation reaction is used as a measure to estimate their antidiabbetic strength\

You can check these two publications for more elucidation on the question.

Using a specific target, such as inhibiting amylase activity, gives you just that... it inhibits amylase!

As far as hyperglycemia is concerned, you have too many factors to consider: efficient breakdown of complex carbohydrates, absorption, utilization, and on the other end of the spectrum, gluconeogenesis. 

I am uncertain that if any single or even if a combination few specific measurements would suffice. If mice are not to your liking, you could always use rats.  ;)

You can access antidiabetic effect of plants extract by using BRIN-BD11 cell line, which is one of the  prominent cell line for insulin secretion studies.

Viishal Musale please can you forward to me your email address. I need to make some inquiries from you on the BRIN-BD11 cell line please. I am expecting it.

There is nothing in established cell lines that really replicates behavior of beta cells, much less islets, including various insulinoma cell lines such as RINm5F or its derivate BRIN-BD11.

However, if one is specifically interested in a specific aspect of mechanism (glucose absorption, utilization, glucose responsive insulin secretion etc.), some of these cells can be used as a tool of preliminary assessment for that specific aspect of "diabetes" - bearing in mind that "diabetes" and "anti-diabetogenic effects" are rather broad terms.