I am doing research for a subscription based game and I'm having a hard time finding how Gamespark can facilitate me in (automatically) checking whether a player has paid this month by checking an external DB.

Is this possible?

 I'm quite sure it can be done easily. 

You can add cloud code to teh built-in AccountDetailsRequest request. The code will call an external web service using:   

var httpSender = Spark.getHttp("https:\\yourWebService....");
var response = httpSender.postJson(customData); //or any other post/get methods offered by GS

    and send back the response to the client. You can also store the subscription data in you own MongoDB collection.

Sounds perfect, thanks!