Even if you are new to the Raspberry Pi and think you will only purchase one, label the microSD card. I simply number them, 1, 2, 3… and record the information about the card elsewhere.
A silver marker is your friend.
I keep track of OS version, installed applications and fixed IP address in an Evernote entry.
Trust me – a year from now when you are wondering what SD card has what application, you will be glad that you did.