look at the left side of the picture, lots of colour and vibrance, contrast is pretty good (lines seem well defined and light and dark is not blurred into a grey mush..)
then look at the rider, very VERY grey, do grey leathers even look that colour deprived?! then look at the forearms, the red gloves have good contrast and the forearm is just a crappy mixture of greyish tones
(this is usually what happens when you mix two different pictures, they are exposed different and have different white balances etc etc)
im not an expert and i dont have the patience to do a serious investigation, but i have made alot of joke fake pictures and have learnt alot about what makes them obvious fakes and how to try and hide that fact
biggest thing is the picture resolution, if you do the work using only very high resolution pictures and then make the end product medium or low resolution its near impossible to notice if its done well
at the end of the day it doesnt matter wether its a faked picture or not, its if you believe its possible to do that on that bike and wether or not a photographer just happened to be there to capture the miracle moment.. lol