Check the OD600 before and after sonication; alternatively, you could check cell counts under the microscope. The latter method is more cumbersome, but it will tell you whether the residual OD600 is due to intact cells or to debris and/or to inclusion bodies

I usually plate an aliquot from before and after sonication. Any growth from after sonication means you still have living cells, then you shoudl either sonicate longer or at higher intensisty.

If you look at the sonicated cell suspension under the microscope, you should not see any intact cells, only small pieces.

One more simple way, it just add 2ul of cell suspension before and after sonication into 20ul of Bradford reagent, observe the color change.