Android - How to find app based on package name?

The easiest way, without using any specialized tools is just to visit that app's page in the Play Store (website, not app).

All apps in the Play Store have their package name as part of their URL, so you can just take your package name and prefix it with https://play.google.com/details?id= to get

https://play.google.com/store/apps/details?id=com.stackexchange.marvin

where com.stackexchange.marvin is the package name which will take you to the page for Stack Exchange Android app.

The question was asked back in 2012 but given that no Android version constraint is given I would like to suggest a solution, which as of now is helpful for Android 4.1 and above.

Install Applications Info from Play Store or F-Droid. Launch it, start searching your package name and your app would show up.