In working with a customer yesterday, I came to discover that in 1902 a feature was added to the Journal Entry API to allow for a test posting (much like simulation in the app). I've been personally asked this question many times in the past by customers so thought it would be good to share. You can find the field TestDataIndicator documented on the API in the hubb. Here is link to synchronous API (the field is on the asynchronous API too).
"The field is used to simulate postings and returns a message. The value is restricted to or . If no value is transferred, is the default value."
In terms of a response, you will basically get a blank JournalEntryConfirmation but the Item log will indicate that the posting would be successful or an error message:
<Note>Document check - no errors: BKPFF $ 0M21EPT</Note>