how do we figure out whether a ligand is weak or strong?

There is no rule or deciding factor for this.

U would need to memorize tge spectrochemical series for the Coordination compounds in priority order.

From weak to strong order ligands.

The increasing Order is:

I- then Br- then S2- then SCN- then Cl- then NO3- then N3- then F- then OH- then C2O42- then H2O then NCS- then CH3CN then Py then NH3 then en then bpy then NO2- then PPh3 then CN- then CO.