How do you check if a specific bit is set in C?

AND the value with a mask that has only that bit set, then test for non-zero: if (flags & (1 << n)) is true exactly when bit n is set.

if (flags & (1 << bit_position)) {
    // Bit is set
}

// Or with named constants:
// bit 0
#define FLAG_READ  0x01
// bit 1
#define FLAG_WRITE 0x02
// bit 2
#define FLAG_EXEC  0x04

if (flags & FLAG_EXEC) {
    // Execute permission is set
}

How it works:

// 00000101
flags = 0x05;
// 00000100 (FLAG_EXEC)
mask  = 0x04;
// Non-zero = true!
flags & mask = 0x04

// 00000011
flags = 0x03;
// 00000100
mask  = 0x04;
// Zero = false
flags & mask = 0x00

Get the actual bit value (0 or 1):

int bit = (flags >> bit_position) & 1;
// or
int bit = (flags & (1 << bit_position)) ? 1 : 0;