One way would be to understand the designer. Once we've done that we can then say what we would expect that designer to do, and test to see if that is what we find. So, perhaps something like "I am the alpha" appearing at the start of all DNA sequences.
This is exactly what ToE does and is subject to testing. It defines the designer - mutations coupled with selection (to simplify), and then uses that to make tests of what we would expect to find.
However, the ID movement refuses to actually define their designer, rendering testing for its involvement impossible. The only part of their designer they seem willing to define is 'intelligence'. We don't know what kind of intelligence we should find or what the goal of this intelligence was. As such we cannot detect if that kind of intelligence was involved.
ID does things backwards. It says, if something cannot be explicitly explained with natural explanations, it must have been guided by an intelligent agent. Just like Lightening was Thor. This leaves ID with the position of 'since we don't know exactly how x occurred, it must have been our explanation.'
Until we actually have a description of our intelligent designer I don't think it can be tested, anymore than evolution could be tested before a description of the natural designer was formulated.