Just paying close attention while normally playing, looking for anything suspicious from a different texture, spotting a shootable switch across the map or a sound cue. If that doesn't find all secrets, then comes the automap. Look for anomalies, differently colored walls suspicious empty spaces. If any secrets still remain, walk through the level again and try the less common secret methods, using any decoration object, shooting most walls etc. If all this fails, proceed to the last resort - wall humping everything madly while shooting wildly everywhere and insulting the map maker for being such a horrible person for exploiting your weakness of having to have 100% secrets and kills on any map you play before ragequitting the map only to repeat the process the next day all over again whichsucksespeciallybadonlargemapsthattakeforevertogothroughormapswithsectionsthatcloseoffpermanentlyonlytowastehoursofyourlifetryingtofindthatonestupidsecretthatcontainsonestimpackandanarmorbonusGRRRRRRRRRRR. (Draws breath: "Yes, I have a problem.....")