Distinguishing cis and trans positions
The way I justify it relates to the main plane of the complex: 
referring to your compounds, the plane I am talking about it the one which includes Co, the two NH3 and ethylenediamine. 
If you want to "join together" with a line the two chlorides, you have to "cross that plane", therefore your compound is trans. 

In the other case (the compound on the right), the plane includes Co, ethylenediamine, chloride and NH3. If you want to do the same thing, i.e. linking the two chlorides or the two ammonia together, you don't cross that plane. In the end, your compound is cis.

Also, take a look at this website as well.
https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_(Inorganic_Chemistry)/Coordination_Chemistry/Properties_of_Coordination_Compounds/Isomers/Stereoisomers%3A_Geometric_Isomers_in_Transition_Metal_Complexes_II

Feel free to add more questions about this!