How to compare a number with its HEX value representation in char

Is there an existing function to compare a number (0-255) with a hex value that is written in chars, such as

char value_in_chars[] = {'A', '2'};
byte number = 0xA2; // or number =162;

?

Or is the only solution ASCII arithmetic?
To make matters worse, I can not be certain if the value_in_char only contains upper case or maybe also lowercase letters.

Background:
The value_in_chars is the checksum of a NMEA0183 sentence which is appended to the sentence, hence it's of the type char. To verify the sentence, I calculate my own checksum and get the result as a number.

Found the same question here, solution was also using ascii math.

So I guess I will do it also this way.