The percentage meter of completion doesn't show until you found an asset in the Star System. So no way to know which have or not.
So do all have some assets to collect, or some don't?
Mass-effect – Are there assets to collect in EVERY star system
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Related Solutions
This strategy lets you avoid reapers while still giving you lots of time to get your scans done.
Enter into the system you want to scan, scan twice wherever you want to (I try to be systematic but it's up to you). On your third scan the Reapers will appear, so it's helpful to be near the edge of the system. Before you hit that third scan, look at where you would like to reappear (top, bottom, left, right). Scan and move out of the edge of the system bringing up the galaxy map. If you're reasonably good at avoiding Repears you can probably get a few extra scans in before leaving the system.
Remaining inside the ring, point your ship in the direction you want to re-enter the system. For example to show up next to the Mass Relay you would point your ship right.
You will re-enter far enough away from the reapers to perform more scanning. Repeat this technique until you've completed your scanning. Note that the reapers don't move closer to you while you are examining a planet/etc so you do have time to land and get your probes launched.
This video demonstrates the technique effectively.
In general, important people and items stand out from the background. If at any point you get lost, pressing the map button (on pc, this is V) can help find your way. Additionally, important people in friendly areas (like the Citadel) are often talking over the crowd loud enough to catch players' attention.
While in a combat zone, this will highlight a point of interest such as a quest objective.
In a friendly area, it will bring up a map with all nearby people of interest listed)
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Best Answer
No, there are quite a few systems that have nothing to be found in them.
The most obvious example is the system with the Citadel in it.
There's a Scanning Guide that is usually linked whenever we start talking about system scanning, but I'm not 100% convinced it is true and complete. It's certainly correct about a few of the systems though, such as the Horsehead Nebula, which contains nothing scannable.