It actually depends on the Pichu you're using. No Pikachu says (Evolves from Pichu) on it, so it depends on the Pichu.
Here are three examples.
Example 1
This Pichu says 'Evolves into Pikachu'. You can evolve it if you haven't played the Pichu this turn.
Example 2
You can use the Poke-Power Baby Evolution to evolve the Pichu, even on the turn you play it. Please note, since it is a Poke-Power, it can be negated with things like Power Spray.
Example 3
you're out of luck completely; this Pichu does not evolve under any circumstances.