You will see it by the players with "chemistry" are highlighted green on the team selection screen.
For the front row it seems you only need three players in their preffred position (i.e not 2nd or 3rd) who have a high(ish) rating
Locks
on't know
Back Row: I used to think you needed three players with complimentary skills(That pic in the corneer by form and morale) but I'm not sure that they do if they have high ratings.
Half Backs: Just a good pairing(can also stretch to 12 on occasion if the 12 is a secondary 10 eg Catt, Giteau etc)
10,12: I'm not sure what this is dependant on.
12,13
Can start at 10) Not sure though I would imagine it needs a Henson/Shanklin or D'Arcy/O'Driscoll(from 2006 or 2007 6N) combination.
Back Three: Different skills-look at the Ireland team in the game to see what I mean(Kickie/Horgan/Dempsey). This is very hard to get- even if you have a back-three of S.Williams, Habana and Latham it would not have good chemistry because they don't have three different skills.
BUT team chemistry appears to have no affect on performance.
Hope that helps