CH3CHO and C6H5CH2CHO can be distinguished by:
Iodoform test
Benedict's test, Tollen's reagent test, Fehling's solution test will be positive for both the given aldehydes. Only iodoform test can be used to distinguish the two. Acetaldehyde will be converted to formate anion and iodoform.
Only acetaldehyde, methyl ketones, and compounds that could be oxidized to methyl ketones give positive haloform test.