Can refractometers measure a banana's glucose content?
Iodine can be used reliably to measure starch content in bananas and it is correlated with sugar (soluble solids).

Summary. A starch staining technique using pictures to rate starch disappearance has been developed to determine banana pulp maturity. The disappearance of starch from the pulp shows linear correlation with peel color (r = 0.76) and soluble solids content (r = 0.81). Pulp pH shows a poor correlation with starch disappearance (r = 0.38). Staining banana pulp starch with an iodine solution is a quick and easy method for estimation of pulp maturity.

http://horttech.ashspublications.org/content/3/3/338.full.pdf