You would locate the source of the leak the same way a tech will....
"looking closely" has it's rewards. A radiator pressure tester would likely
help, but, even then, looking will be the answer....

There is nothing magical when it somes to auto repair... it is a matter of
logic along with cause and effect.....

Others seem to find nothing wrong with driving for extended periods of time
with obvious concerns. My loving bride is often required to drive long
stretches on a lonley highway (in a nasty climate, I might add) and I can
ill afford to have her drive something with a known concern. For any
concerns, I employ the same technique I use in my daily routine......