How to proceed, if I get wrong answer for my submission, after basic test cases get passed

I am using JAVA to code here, I have encountered this situation several times, so asking,

After coding the problem on my IDE and getting the basic test cases provided in problem, successfully getting passed, I submit the code.
But it gives me wrong answer or partially correct answer.
My understanding is that there is some test case, which I am not able to think of , is not getting passed.

But how do we proceed further in such situations ?
Example: SPREAD2 in SnackDown Qualifier round.
My code:

//to determine in how many days information will be spread out 
/*ex: array:
2 1 1 5 5 5 5
will take 2 days to spread info to all 7 people*/

import java.util.*;
import java.io.*;

public class Main {

public static void main(String[] args) throws IOException{
	try {
		Scanner sc = new Scanner(System.in);
		int t = sc.nextInt();
		
		while(t-->0) {
			int i=0;
			int size = sc.nextInt();
			int[] arr = new int[size];
			
			for(i=0;i<size;i++) {
				arr[i] = sc.nextInt();
			}
			
			int count=0,sum=0,temp=0,rem=size;
			i=0;
			while(rem>0) {
				sum=0;
				for(int j=0;j<=temp;j++) {
					sum += arr[j];
				}
				
				i += sum;count++;
				rem = size-i-1;
				temp = sum;
			}
			System.out.println(count);
			
		}
	}catch(Exception e) {
		System.out.println("exception is "+e);
		return ;
	}
}
}

You can use "Random Function " (like rand() in c++) to generate some random tests . Then cross check them on paper whether your code’s output is correct or wrong .

I hope it will help.

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