Oops! Thanks Petr, that is correct, there is a way to do this after all I had just forgotten that method myself!

To use the script Petr provided above, go to Options / Shortcut keys, and add in a new entry.

Then add whatever key you would like, and paste in the script above as the command.

Then when you hit that key, any closed solid will be selected, that is a method you can use currently to determine if an object is a solid or not.

Sorry I forgot about that method!

- Michael