Uusually, dressing for seaweed salad is shiso dressing.
Dressing is made from Shiso, Soy sauce, vinegar、and sugar.
Here is basic information of Shiso from wikipedia
Perilla frutescens - Wikipedia, the free encyclopedia
I think some contain katsuo bushi(dried fish).
Siso dressing has same colour as soy sauce, so it won't be difficult to identify.
However, I am not sure about outside of Japan.
Here in Vancouver, restaurants use different dressings.