Visual chain identification
With accurate vernier calipers or a micrometer, measure the width of the outside plates.
https://en.wikipedia.org/wiki/Bicycle_chain#Width shows this table of chain widths.




Speeds
Width




6
7.3 mm (9⁄32 in) (Shimano HG), 7.1 mm (9⁄32 in) (SRAM, Shimano IG)


7
7.3 mm (9⁄32 in) (Shimano HG), 7.1 mm (9⁄32 in) (SRAM, Shimano IG)


8
7.3 mm (9⁄32 in) (Shimano HG), 7.1 mm (9⁄32 in) (SRAM, Shimano IG)


9
6.5 to 7.0 mm (1⁄4 to 9⁄32 in) (all brands)


10
6.0 to 7.0 mm (1⁄4 to 9⁄32 in) (Shimano, Campagnolo)


10 (Narrow)
5.88 mm (7⁄32 in) (Campagnolo, KMC)


10 (Narrow, Direction)
5.88 mm (7⁄32 in) (Shimano CN-5700, CN-6700, CN-7900)


11
5.5 to 5.62 mm (7⁄32 to 7⁄32 in) (Campagnolo, KMC, Shimano CN-9000)


12
5.3 mm (13⁄64 in) (SRAM)




So if your chain is 5.8mm wide then its probably 10 speed, and 5.6mm is likely 11 speed.   The difference is small, 0.2mm is about 8 thousandths of an inch, you're not going to see that on a ruler.
However if you had known 10 or 11 speed chain laying flat on a tabletop, and your current chain flat beside that, you should see a visual difference, or at least feel one with your fingers.

Another idea is to look for a quicklink or master link.  Many of them have 10s or 11s stamped into the sides.  You're unlikely to get an 11s master link to close on a 10s chain, but the opposite is certainly possible.