Can aflotoxin presence be inferred from the mild discolorotion at the middle parts of a hazelnut?
Depends on who you bought them from and how you stored them.
Molds can grow in nuts and nutmeats and produce mycotoxins (from toxin-producing strains such as aflatoxins by Aspergillus flavus). The mold is usually visible and below is picture of Aspergillus flavus growing on a nut product. Source

The amount of aflatoxin produced by the mold is dependent on weather conditions and storage. We can only account for or control the storage conditions. When food substances are not well preserved, fungal growth may occur, leading to the production of mycotoxins.
The pictures presented could also just be an improper storage (with no health risks). There's indication that storage of wet nuts in poorly ventilated silos increases the potential for developing kernel brown centers. Source
If these nuts were stored in improper conditions from farm to store and the farm has been associated with a breakout of Aspergillus flavus then there's chance, but slim. QA/QC and aflatoxin levels are regulated. Codex Standard 193-1995 mentions the maximum levels of aflatoxins and sampling plans.
If the nuts had mold on them and they were washed to get rid of the surface mold, they could have spores from Aspergillus flavus capable of producing alfatoxin. Aspergillus fungi can case discoloration of  certain grains (rice), but that implies that you're able to see them on the product (the mold growth). The nuts in the picture seem quite clear.
In terms of where you bought the product, street vendors are at higher risk of having their products present alfatoxin problems rather than commercial enterprises, because of the location the product is sold at. Source