To read a two's complement number, check the leftmost bit. If 0, treat it as unsigned binary. If 1, it is negative: flip all bits, add 1, then negate the result.
Example: What is in 8-bit two's complement? Leftmost bit is 1, so it is negative. Flip: . Add 1: (which is 5). Negate: . Result: .
Shortcut: the leftmost 1 in a negative number is worth , and the rest are positive powers. For : . With practice, you can read two's complement as fast as unsigned.