By visually checking the rod from time to time and seeing if there is any slop, and seeing if it is stretched or not.
Normally you would put the piston right to the top, and then gently rocking the crank to see if it moves around. Normally it does a little bit, you can't stop that, but it depends on how much the crank moves without the piston moving.
If you think it's too much, replace the rod and piston pin.
I've heard of people replacing rods after 5 gallons after the engine is completely run-in, but it's their words against the world, and it depends if you are racing hard or just having a bash.