After enabling Alt Image, you need to wait for the Image Optimize to process all the Image, the Estimate time is about 10 hours.
There are 2 ways to check:
# Method 1: Check in the Storefront
You can check the Alt image in the Storefront, but some theme does not support the Alt tag to show in the Image.
Go to your storefront, right click anywhere in web content area, then choose Inspect.
Find an image in the content, if you see an image with alt as you configured.
# Method 2: Check in admin
You can also check in the Admin->Products->All Products and Inspect Element.