Useful means 'what can we learn from it'.

If you are evaluating HOW USEFUL something is you are asking

What does it tell us?
What doesn't it tell us?
and you are considering
Whether the nature of the source LIMITS its usefulness.

Of course how useful something is will depend on WHAT YOU ARE USING IT FOR
so you need to consider USEFUL FOR WHAT?

Think
POSITIVE
NEGATIVE
POSITIVE.

Begin: Source X suggests........ then pick out the key ideas or information from the source.
THEN
MATCH this against your OWN KNOWLEDGE.
THEN
Consider whether the usefulness of the source is affected by the
Ws
(who, what, when, where, why, who for?).

FiNISH WITH A STATEMENT ABOUT 'USEFUL FOR WHAT'.

For example a British cartoon will tell us a great deal about British perceptions of an event. It may not really give an accurate account of what actually happened.

A Presidential speech will tell ou a great deal about what the President is trying to convey to the American people but it doesn't necessarily tell us everything we want to know about the attitude of the American people.

§ What does it tell us? § What doesn't it tell us? § Does it trick us by adding something in? § Is it a composite image (ie lots of ideas put together for propaganda purposes?) § How representative is it?

Rather than just vaguely saying a source is biased you can say

IT IS ONE SIDED
OR
we need to treat it with caution.

NEVER REJECT SOMETHING AS TOTALLY USELESS.