A fly on the wall....

[laepelba]

Is there a similar saying in Spanish?

"A fly on the wall...." is used to imply that one would love to have been present to overhear a conversation.

For example:
I would have loved to be a fly on the wall when Sue told Jack that she would no longer write the math competitions. I can't imagine how he would have responded to her!

[Perikles]

My dictionary claims there is no equivalent image, just

me habría gustado estar allí cuando ..... :(

[AngelicaDeAlquezar]

"Me hubiera gustado ver la cara de Jack cuando Sue le dijo que ya no escribiría los concursos de matemáticas."

[laepelba]

Thanks, both of you! :)

[poli]

I don't think there is a common phrase in Spanish with the exact meaning.
"I wish I had been a fly on the wall" would mean "me gustaría haber sido un testigo invisible"