How To Recognize Buffer Underwrite Vulnerabilities

The following is an example of code that may result in a buffer underwrite, should find() returns a negative value to indicate that ch is not found in srcBuf:

int main() {
... 
strncpy(destBuf, &srcBuf[find(srcBuf, ch)], 1024);
...
} 

If the index to srcBuf is somehow under user control, this is an arbitrary write-what-where condition.