I've owned both and they look different but I honestly can't explain the difference (I don't care). the best way to tell the difference is to drive the 2.5L and then you'll realize the struggle is over

 

True but have not driven a 2.0 to know the difference.

 

2.0L has cam advance sensors on both intake and exhaust cams ( 2 on left front of engine cover, 2 on the right )

2.5L only has them on the intake cams ( 1 per side )

 

2.0L has cam advance sensors on both intake and exhaust cams ( 2 on left front of engine cover, 2 on the right )

View attachment 306252


2.5L only has them on the intake cams ( 1 per side )

View attachment 306253

I have a 2021 Limited and it has 4 cam sensors. I also have 2016 Outback with the 2.5L and it has only two as shown in your photo. The 2021 Crosstrek has FB25D (with direct fuel injection). While your photo of the 2.5L is the FB25B (non direct fuel injection) - my Outback has that engine. Externally the 2.0L and the 2.5L in the 2021 Crosstrek should look the same externally. List of Subaru engines - Wikipedia

 

The engine blocks are identical, but the fastest way would probably be the engine stamp on the block itself which should be marked FB20 of FB25. And when I say identical, I mean Subaru literally just bores out the block and puts larger piston heads in. I don't think the stroke is even different, they use the same connecting rods.