General

Does the diagram have a title?
Do you understand what the diagram type is?
Do you understand what the diagram scope is?
Does the diagram have a key/legend?

Elements

Does every element have a name?
Do you understand the type of every element? (i.e. the level of abstraction; e.g. software system, container, etc)
Do you understand what every element does?
Where applicable, do you understand the technology choices associated with every element?
Do you understand the meaning of all acronyms and abbreviations used?
Do you understand the meaning of all colours used?
Do you understand the meaning of all shapes used?
Do you understand the meaning of all icons used?
Do you understand the meaning of all border styles used? (e.g. solid, dashed, etc)
Do you understand the meaning of all element sizes used? (e.g. small vs large boxes)

Relationships

Does every line have a label describing the intent of that relationship?
Where applicable, do you understand the technology choices associated with every relationship? (e.g. protocols for inter-process communication)
Do you understand the meaning of all acronyms and abbreviations used?
Do you understand the meaning of all colours used?
Do you understand the meaning of all arrow heads used?
Do you understand the meaning of all line styles used? (e.g. solid, dashed, etc)