How can you tell the difference? reducing agents undergoes oxidation thus they lose negative charge while a lewis base donates electrons so it itself loses electrons?
an oxidizing agent is atom/molecule that gets reduced because it gains electrons. You can think of an oxidizing agent as an electrophile, since electrophiles want to gain electrons.
A lewis base is the exact opposite of this. Lewis bases want to DONATE their electrons, and thus they are nucleophiles. A nucleophile is anything that has lone pairs, or a pi bond (since pi bonds have electrons that they can use to attack electrophiles)