Can I see a list of gyms I left a pokemon at?

If you keep personal track of which Pokemon you left where, then yes. The game shows a colored icon above one of your Pokemon if it's at a gym, but not which gym it is. You could also monitor which Pokemon you have at 1hp. Additionally, the icon in the top-right of the Shop (which you use to claim rewards for holding gyms) tells you the number of gyms you control.

A good little trick is rename your pokemon with the gyms location.

Change:

"Ponyta" to "Town Hall"
"Rhydon" to "College University"
"Rattata" to "University Cafe"

This way when a gym you own is taken down, you know which one it is.

As of today, there is finally a way to see what gyms you have Pokémon occupying!

If you go to your trainer profile after visiting gyms, there will be a list of gym badges you've earned. There is an indicator next to the gyms that currently have one of your Pokémon occupying it:

If you tap on this gym badge, it will show what Pokémon you have defending that gym as well as how long that Pokémon has been defending the gym for.