Farah was born 23 March 1983. These are his PB's through the end of 2005, his age 22 season, from Tilastopaja.org.
800m: 1:48.69, 3rd place, 3 Aug 2003
1500m: 3:38.62, 7th place, 19 Aug 2005
Mile: 3:56.49, 1st place, 8 Aug 2005
3000m: 7:54.08i, 6th place, 5 Mar 2005
2 Mile: 8:36.73, 8th place, 21 Aug 2005
5000m: 13:30.53, 3rd place, 25 Jun 2005
10K road: 28:58, 3rd place, 28 Nov 2004
Reads like the PB's of a top-tier NCAA athlete, but just short of NCAA individual champion quality (although the quality of the NCAA wasn't quite as good in 2005 as it is now; you could possibly have been an NCAA champ at the time with those PB's).
Farah has PB's on Tilastopaja.org dating back to 1999, when he ran 8:21.25 as a 16-year old. So he'd had 6 years of training and competitive buildup to setting the list of above PB's.
You have to answer the next question based on what you think is physically possible within the sport. Is it possible for someone with Farah's athletic background to become a dominant global champion within 6 years through completely legitimate means?
Impossible to say for sure, barring a positive test. But if Farah is clean, it must be one of the latest instances ever of that kind of talent shining through.