My idea is to use some type of symbology or highlighting of the layer name to indicate that there is a definition query applied to a layer when viewing it in the TOC in ArcMap. It's easy to forget that one is in place and it's cumbersome to have to go into the layer properties to check. An off/on toggle for the definition query would also be helpful if you have a useful query in place but need to turn it off occasionally to access the additional data. This is better than having to either copy and repaste the query or create the query again from scratch.
If interested in the ability to toggle on/off or use a pick list of definition queries on a layer, go to Make Definition Query able to be turned on/off with Check Box