Hi natty1905

I think it is both about observing the movement of the scapular first then testing the muscle. It sounds like you are reading good texts. Shirley Sahrmann gives an excellent description what to observe the scapula doing during shoulder flexion or abduction on pp 209-210 or her book. So that is the observation part. In Kendall's (5th edition) book the test with the shoulder flexed to 120-130 degrees then palpating the inferior part of the scapular. I test it by providing a resistance to protraction of the lateral border of the scapula just above the inferior angle.

Once you have clinically seen a patient with weak lower serratus anterior it is easy picture to remember. However to really get the idea you might need to go through it with an experienced clinician - It is a bit hard to explain in great detail in writing

If you are still finding it confusing try Donatelli’s book - slightly different presentation and testing but achieves the same result. I haven't got my copy on hand but as I remember it also has an excellent CD that goes with it as a visual guide.