As an e-fff file is based on the ubl 2.0 format, it is possible to validate the document structure with an XSD validation. You can find the xsd at this address:

Philip Helger has created an online validation tool to validated all kind of xml files and amongst them e-fff

It is also possible to use the online tool to quickly see if the file meets the minimum criteria of an e-fff file. This does not yet mean that the document will be accepted by all accounting applications. (Please check with the list of partners to see if your accounting software is listed)