How can I determine:

• a system’s trade value?
• which trade routes the system contributes to?

The latest patch notes include this change:

Reworked trade info to display accurate information

... but I’m not sure how to use the information to evaluate potential trade subsidiaries or routes. The previous version seemed to answer the first question (what’s the trade value?) in scan view, but I couldn’t figure out exactly how the systems contributed to the routes. The new interface shows trade efficiency but I have no idea how to interpret that, and the tooltips don’t help.

I apologize for not seeing this sooner, hopefully you'll still check back and see the answer and I hope this helps. 

A system's "trade value" is simply the modifier controlling the trade routes running to and from that system, it's not a number you'll actually see in the game anywhere (to my knowledge). 

You can see which trade routes a system is contributing to by checking in the Economy Screen, to the left of the screen you'll see a list of your headquarters and your subsidiaries. All subsidiaries in your empire trade with all headquarters in your empire. If you make a trade agreement with another player, you'll see their subsidiaries listed in the "External Trading Subsidiary" list at the bottom, these are also trading with your headquarters and have no relation to your subsidiaries. 

That is the only information I can find, hope it helps.