Adalia decempunctata vs Harmonia axyridis

The size difference is usually clear enough to know which one you're looking at (compare this image):

Nevertheless, from photo, when size is not clear, the two can be hard to identify.
The collage above shows the differences in locations of the spots, but the spots can be partially or completely missing on both species, making it harder still(!)
In that case look at the pattern on the _head_ (not on the pronotum!) to see which is which.