The default setup for AEM that is used in most installations is TarMK. If you are not aware of which one your setup is using it's most probably TarMK. You would certainly know if it was MongoMK as you would need dedicated MongoDB cluster infrastructure and someone running and managing the Mongo part of it. Also there are only a couple of use cases where it would be recommended to use MongoMK, the default TarMK is the recommendation for the majority of projects.
If you want to be sure, please refer to the "AEM Configuration" section on this page of the documentation  and verify if any of the mentioned configs are present on your environment. You are running MongoMK if:
Your AEM instance is running with the following run mode: "crx3mongo"
You have an active configuration of the DocumentNodeStoreService (org.apache.jackrabbit.oak.plugins.document.DocumentNodeStoreService) with connection details pointing to your MongoDB infrastructure.
You don't have an (active) SegmentNodeStoreService configuration (org.apache.jackrabbit.oak.segment.SegmentNodeStoreService) and no according TAR files are present in the file system at crx-quickstart/repository/segmentstore (for TarMK, there would be at least one, probably multiple dataXXXXXX.tar files holding the actual repository data; however, the position of the segment store can vary if it has been configured to a different value than the default in the SegmentNodeStoreService configuration).
Another note on your upgrade plans:
Please re-evaluate if your target version of AEM should really be AEM 6.4. Service Pack 8 CFP 4 is the last CFP for that version of AEM (see Release Roadmap ) and Core Support has already ended (see EOL Matrix ). I highly recommend to use AEM 6.5 as a target version for your upgrade!