It is a really good idea to review progress with your Mentee during every session. In addition to this, you may want to build in more formal review dates at regular intervals.
This is a good time to discuss what the mentee has achieved since the last meeting and this might be against their action plan. It is helpful at this point to help the mentee reflect on how they achieved, the difference this has made to them, whether they can replicate this in other areas and how they feel having achieved. This provides the basis for moving forward in an informed and positive way to further achievements.
It is also useful to discuss what the mentee hasn't achieved and the reasons that prevented them meeting their outcomes. If you are using the GROW model, it is a good time to re-visit the Reality section and ask how realistic they had been in setting those actions in their plan! The discussion should engender confidence and realism in the mentee to enable them to set more realistic goals.
If you use the PSOR template for you and your Mentee to record your sessions it has an inbuilt review section. The document should remain confidential between mentor and mentee. It is based on the Self Organised Learning Approach of Laurie Thomas and Sheila Harri-Augstein
Suggestions of subjects you should cover in the review can be seen here.