I check often and change when needed. I can either smell it (and I don't mean it stinks but I know the smell of my kids wee) or feel through the clothes that it feels warm or full. By full I mean that I can feel the fabric feels different, more dense due to the wee.

With cloth it's worth investing in some AI2's because the beauty of them is you can just replace the boosters if they are just wet (obviously not for a pooey one or soaked through to the shell). Itti Bittis for eg cost about $25 for the nappy and $12 for an extra booster set so having a few of them spare is like having another nappy for half the price