I am not aware of an optical telescope capable of showing proof from earth. Part of the reason is the flags are pretty small and it's a very long distance. However, you can see the flags or what is left of them and the landing sites using images from the Lunar Reconnaissance Orbiter (LRO).
The LRO was able to use it's camera to document all of the Apollo landing sites. In 2011, Eric M. Jones collated the images to show the conditions of the landing sites and evidence of the condition flags was presented.
Buzz Aldrin indicated he saw the flag knocked over on liftoff from the moon.
This quote was from the manufacturer:
Dennis Lacarrubba, whose New Jersey-based company, Annin, made the
and sold it to NASA for $5.50 in 1969, considers what might happen to an
ordinary nylon flag left outside for 39 years on Earth, let alone on the moon.
He thinks for a few seconds. “I can’t believe there would be anything left,”
he concludes. “I gotta be honest with you. It’s gonna be ashes.”
Apollo 14 and 15 had no evidence from the images of the flags. It was thought they may have been destroyed by the exhaust gases on liftoff.