Any single note played backward has similarities and significant differences from the same sound played forward. This because there are several parts of a sound that we don’t consider. A sound is considered to have an envelope (you can actually graph them). They start with the attack, there is a peak, there is some amount of sustain and then there is decay.
If you were to play a single note on the guitar and record it and play it backwards you would hear decay, sustain, the peak and the attack (which sounds very abrupt played backward). Only the “sustain” part of the note (steady state without a lot of dynamics) sounds the same forward and backward for most people.
This is completely different from reading the notes on a piece of music from bottom to top and right to left and playing them that way. The sound envelope for each note remains the same and it might be possible (for some) to recognize.
This technique is called backmasking and you can find some information on Wikipedia. There is also a website here that will allow you to select a segment of familiar songs and hear them forward and backward.
Some of what helps in the identification is a familiar voice or orchestration (listen to Eagles Hotel California) at the link above for example. The new rhythm introduced into the clip by playing it backward is hearing the attack segment of the sound backward with a sudden end.