How to know if something will be a good oxidizing/ reducing agent? (Besides the activity of the metal) One of first redox reaction I was taught in school, is probably single replacement. It is easy to predict cause all you need is a table of metal activity series.

But then there are more complicated or less well-known agents. Like permanganate ion, which is used commonly in determining the purity of iron.

For agents like this, how do you know if it is strong enough to reduce or oxidize the other elements? 

(Is there also a table for this?)
The relative strengths of oxidising and reducing agents can be inferred from their standard electrode potentials. The strongest oxidizing agents are shown from the standard electrode table.