When I open a old project in a current designer, say v5.6, is there a way to see the version that the project was last saved by?

When you save the project file again, it will tell you that it's saved previously in e.g. v5.1 and you're now in v5.6. You can then cancel it.

It's also in the xml in the project file itself, at the top.

<?xml version="1.0" encoding="utf-8"?>
<!--LLBLGen Pro v5.6 project file. Do not manually edit this file. Incorrect references could lead to an unloadable project file.-->
<Project Version="5.6" TargetFrameworkName="LLBLGen Pro Runtime Framework" UseLegacyDefaults="false">
...

As a designer feature request: It would be useful (to me at least) to have this available before the save attempt. Maybe under Project -> Properties or right click on the project tree node on the left.

My use case is the a have a series of projects last saved under various versions for various clients.

The client is asking for a quote on a change or I might just be trying to plan. It would be handy to know where I would be starting from before ever deciding to update the model and save it.

Good point. Will add it.

In v5.7 the format version is visible next to the filename in the statusbar.