No one can tell from a picture whether its taken by a Nikon/Canon/Pentax/Olympus,etc..
Perhaps if you use an entry level 5MP dslr versus a Hasselblad Medium Format 60MP, you can see the difference in resolution when you blow up to billboard size.
I'm no pro and only indulge in photography as a hobby. I use Nikon, Fuji, Hasselblad and Leica film and digital cameras.
My dslrs are Nikon & Fuji. I use Nikon only because I started with Nikon and have got used to their control interface as well as accumulated quite a number of pro lenses.
I use the Fuji SPro range because it uses the same lens mount as Nikon and I prefer their color palette compared to Nikon.
For example when changing lenses, Canon/Nikon bayonet mounts work in opposite directions.
The brand is not the issue, its the type of camera you should choose for the shooting genre you are into for example:
DSLR for telephoto,macro, high speed sports.
Rangefinder for street or quiet and purposeful, artsy shots.
Medium format or even large format for high resolution requirement like landscape, architectural.
Medium format also for high end model/fashion shoots where big blowups are required
I always say that the most useless question to ask in buying a camera is which brand is better.....