This is a tricky question, since people do not like criticism. On the other hand, if they were happy with their digital marketing efforts, they would not look for a new marketing manager.
I would suggest the following: Say that results matter the most, and since you have no access to their analytics or other tracking tools, you can not tell the ROI of their campaigns and therefor it is hard to understand what they could do better.
Say, however, that there is always room for improvement, both on their website and social media, and that once you have access to all data and statistics, you will easily identify room for improvement.
Obviously if something clearly strikes you (their website design is extremely outdated, they update their social profiles once in a month, or they even do not have them), you can point these things out. And you can also praise them for the things they do well in their online marketing strategy.