This was a shocker for me. I’ve played for years and didn’t know about this – so you CAN learn new things all the time in this game!
Go to the Tech Tree and look at the top, left corner. There’s a little icon there that looks like a bar graph. Click on it and it will open a box that shows you all the resources you need for your current Age, how many of those resources you actually have, and how many you still need to collect. There are arrows at the top that let you move forward to see what you need in Ages you haven’t reached yet.
Bingo! An easy way to find out if you’re ready to move on to the next Age or not.