Usually flameouts, burnt glowplugs, erratic running characteristics........rapid tune changes, poor idle...

easiest way to know for sure is to pull the engine apart and inspect the internals, paying specific attention to the crankshaft...As the crankshaft doesn't lie...if dirt passed thru the engine the crankshaft will show it... doesn't matter how good your filter maintenance is, or what brand of air filter you use, if you see these marks it means dirt got in there.....

anyways here is what your crankshaft will look like if your engine has ingested dirt....... Any sort of marking of this nature means foreign material has passed between the crank and the case and been hard enough to scratch the hardened tool steel crankshaft..