Keep track of which member is stored

A union doesn't record which member you wrote last. A separate kind value can do that job. Here a struct holds both the tag and the union, so the program checks whether the ticket carries a tool count or a grade before printing.

#include <stdio.h>

enum Kind { TOOL_COUNT, QUALITY_GRADE };

struct Ticket {
    enum Kind kind;
    union {
        int count;
        char grade;
    } value;
};

int main(void) {
    struct Ticket ticket = {.kind = TOOL_COUNT, .value.count = 4};
    if (ticket.kind == TOOL_COUNT) {
        printf("Tools: %d\n", ticket.value.count);
    } else {
        printf("Grade: %c\n", ticket.value.grade);
    }
    return 0;
}

Try the grade case by changing kind to QUALITY_GRADE and the stored member to grade = 'A'. Change both together. A grade tag paired with a count would send the program to the wrong member.