Round 1 of Stevenson vs. Fonfara was slaughter and could have been stopped by the end. But honestly, I'm not sure what to think of a 39yo who seems to have not declined at all with age. Hell, most of the top dogs are probably on gear but Stevenson, Marquez and others +37yos who seemingly lost no foot or handspeed with age are highly suspect to me at least.
Until boxing decides on an overeaching organization to test all the fights we will never really know.
There is not big overseer seeing boxing as their is with MMA(UFC). There are too many players in the boxing game with differing opinions about how testing should be done. Boxing is still pretty much in the dark ages compared to other sports when it comes to testing fighters.
We cannot have only one big organization such as USADA test all fighters because
a)it is very expensive
b)there are many state athletic commission that do not agree with their methods (out of competition testing all year)
c)Many belt organizations have their own testing agencies that they prefer (WADA,VADA,Olympic style testing)
The only fighter that really was very suspicious was Manny Pacquiao. Up until 2012 he moved up through 8 weight classes and maintained his speed and power which is extraordinarily hard to do.
Paulie Malignaggi was very critical of him stating that if you start testing him you will see a very sharp decline and he was right. After the Marquez fight and up until before you could see he was not the same fighter. Many were saying that he no longer had his killer instinct.
Another strong indicator was the 2010 negotiations between Floyd that fell apart due to Anti Doping Testing. Not to take anything away from him or his legacy though.
Also Stevenson is very dependent on his Power which is usually the last thing to go. Considering that he has mostly stayed in the Light Heavyweight Division and has no plans of moving, I do not see it going away any time soon. It is suspicous that he has maintained it almost in his 40s though.