Non-scientific method:
Well, an easy way is observing the filament under a microscope: if it contains one cell it's unicellular and if it contains more than one cell it's multicellular.
Scientific method:
Filamentous algae are, in principle, single-celled. But, sometimes they can form colonies and evolve to be multicellular. But still, if they just form a colony they'll still be unicellular until they start changing.
Answer:
Unicellular.
Hope it helped,
BioTeacher101