Probably the best serratus/intercostals display ever was on Robby Robinson in his prime(mid-70's)...if ya don't know him do a google. Doing dumbell pullovers cross-bench style hits the serratus well. Pullover machines like the old shcool Nautilus pullover machines hit em too. Leaning side cable crunches and various other side crunches hit the intercostals if ya do it in a way to focus the angle so you feel it up higher than conventional ab/oblique exercises. The serratus & intercostals have that finger-like look when really lean...which needless to say, I haven't seen my mine clearly since...well...my previous life.lol
...these was when he was older near 50...yet you can still see the killa serratus & intercostals development. I cant find any pics when he was at his best in the late 70's. When he hit his vintage 'side crunch' shot...good Lord ...his serratus & intercostals looked like 2 half-racks of ribs overlapped!
the serratus anterior can be trained and strengthend by doing uneven push ups ( 1 hand higher then the other) or dumbell presses that are done unevenly obviously with light weight but with weight in 1 hand and not the other