Hey, How can you tell when you need to change conrods?

2 ways...

1) when there is to much play, with a new rod by hand you must feel if it is the rod or the crankpin.

2) after an amount of liters fuel used. There is also a non-visable wear in the rod on molecule level, the material is getting tired and so loosing strenth, thats why to change it from time to time.
This periode is mostly mentioned in the manual.

What i normally do is;
remove the backplate. put a drop of oil on the crank and conrod and turn a little the crank both ways. When bubbles appear its to much play.

after break in completed according to most it is better to change the rod.the break in process really giving stress to the rod, forcing the piston too get through the sleeve. as for me i change the rod after 1 race. usually the 2nd rod will last longer. if you have doubt its better to change than take risk of a blown rod and causing damage to other parts.