What assumptions? I didn't know the original artist beforehand, so I couldn't recognise the work/style.
I think what Chinatsu means is check the artists other works if any. If you look at akio55's other works, you'll find that they only have one other (pixiv #19126751). It is of such a drastically different style, that it throws up a lot of red flags.
But some of these photoshoppers are much better than others, so you always need to be wary. Yaminokonjiki1 is one such artist (artist #160635), and even I got fooled (post #3059879).
There are much less of these artists on Pixiv than Twitter, but they are still there. Nicoseiga, Tinami, and Nijie are also mostly safe. Tumblr is another source to watch out for.