To distinguish between Acetaldehyde ($CH_3CHO$) and Phenylacetaldehyde ($C_6H_5CH_2CHO$), one must look for a specific structural feature that differentiates them.

1. Iodoform Test: This reaction is specific for compounds containing the methyl carbonyl group ($CH_3-C=O$) or a secondary alcohol with a methyl group on the \alpha carbon ($CH_3-CH(OH)-$).

• Acetaldehyde ($CH_3CHO$): Contains the $CH_3-C=O$ group. It reacts with Iodine ($I_2$) and NaOH to form a yellow precipitate of Iodoform ($CHI_3$).
• Phenylacetaldehyde ($C_6H_5-CH_2-CHO$): The carbonyl carbon is attached to a benzyl group, not a methyl group. Therefore, it gives a negative Iodoform test.

2. Tollen's Reagent: Oxidizes both aliphatic and aromatic aldehydes to carboxylic acids, producing a silver mirror. Both compounds would likely give a positive test.
3. Fehling's and Benedict's Tests: These generally oxidize aliphatic aldehydes. Phenylacetaldehyde is an aryl-substituted aliphatic aldehyde (the -CHO is on the side chain), so it behaves similarly to acetaldehyde in these reduction tests, making them unsuitable for distinction.