All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
In regard to the general ruling, whoever believes in Darwin's theory of evolution is denying Allah and His Messenger . There is no doubt that this is Kufr (disbelief).
Now, we have to look at the ruling on an individual. A specific person cannot be ruled to be a Kaafir (non-Muslim) unless some conditions are met while there are no impediments for declaring him as such, as he might be ignorant or have misinterpreted the texts, for example. The scholars say that if a person’s Islam is proven with certainty, it cannot be overruled except with such certainty.
For more benefit on the conditions for declaring someone to be a Kaafir, please refer to fatwas 8106 and 87963.
It is an obligation not to hasten in declaring someone to be a Kaafir as this is a very serious matter; there is a severe threat regarding declaring a fellow Muslim to be a Kaafir.
Ibn ‘Umar narrated that the Prophet said, "Any person who said to his brother 'O Kaafir' it (the sin of Kufr) will be borne by one of them; if his brother is not as he said, it bounces back to him." [Muslim]
For more benefit, please refer to fatwa 360853.
You should strive to avoid obsessive doubts in this regard and not pay any attention to them.
Allah knows best.