From within the main page of the Trusted Advisor dashboard, any alerts and warning will be displayed. From here, I can select the yellow warning which is listed as shown:

We can see that the issue relates to the VPC Network Interfaces check and that one of the 15 items that this check performs a review against is breaching the 80% usage of the service limit within my account. To gain more information, I can click on the expand arrow or the title of the check. This then presents the following:

Now I can clearly see where the issue is. Within the eu-west-1 region, I currently have four VPCs configured and the service limit on a VPC in any region is five, hence the 80% alert. Now that I am aware of this, I can plan accordingly if I need many more VPCs within that region. If I another two or more, then I would need to request an increase via AWS, as I would only be allowed one more VPC in that region until my service limit was reached.