Chemistry reducing agents Watch
To tell which is the strongest reducing agent, you can change the sign of its respective reduction potential to make it oxidation potential. The bigger the number, the stronger the reducing agent. For example, among Na, Cr, Cu and Cl−, Na is the strongest reducing agent and Cl− is the weakest one.
So if you have that data to hand that's a great way to tell!
Otherwise, think of it in a different way. For example: a reducing agent is a chemical substance that donates electrons to atoms of other elements. All the alkali metals are strong reducing agents as they have strong tendency of losing electrons and the reducing power increases.
As you already know, reduction is gain of electrons. However, a reducing agent is something which enables reduction of another species, ie. it donates electron. Thus, a reducing agent is oxidised when it reduces something and vice versa. I often find that students can really struggle getting these terms straight in their heads. Apologies is this is like falling off a log for you.