second time through lessons
As I'm going through the lessons for a second time, when I do the challenges, I will go back and see completed lessons in color again (instead of the 'gold' completion color); does Duo grade my performance and then realize that I need more study time in these lessons?
If it's been a while, it might take several strengthen sessions or even redoing all the lessons again to get back to gold.
Here are a couple things that I've found it helpful:
Skill strength viewer. I used it to see which skills are weakest and work on those first. It doesn't always seem to correlate to what's gold or not, but it's a reasonable indication of what I need to study. https://www.duolingo.com/comment/22449961/Skill-Strength-Viewer-userscript
I have two accounts. Since the progress test has been removed, I've used the second account to take the placement test and see how far I'm able to test out. Skills beyond the point need more practice.
I've done reverse trees or laddering (one foreign language from another). As you have to answer more in the target language and a higher level of skill in that language is assumed, it's a good way to find weaknesses that need work.