In the callout Library, we can't see the differences between filled or transparent items (in v.1.3.26, there was a title ; a title is not useful everytime, but it was when choosing between callouts).
This is frustrating me as well
I believe most of the call outs are solid apart from the first 4 in the library.
A quick and easy way to check is to set a different background colour and add the callout.
I couldn't find a filled rectangle when I searched for rectangle. I had to create one myself.
A "quick and easy way"? Easy, yes but it is by no means time effective to search, add, delete, repeat - for every filled element you need before you find one. Then repeat the process again for a new scribe because the elements are not named and you need to remember which one was the filled one.
Yes, I can see that this is frustrating, we are looking at ways to improve the tagging and classification of images and if you have any specific ideas for this please add them as new 'Ideas and features requests'.
What I have found to work is if you search for 'flag' in the image library search box, pick a flag, add to the canvas, go into the element properties of the flag and change the draw type to silhouette (see image).
You can then change the colour of the fill of that image by clicking on the coloured square in the bottom right of the preview window.