By What process will a scientist arrive at a 'logical' conclusion? (when UFO's are sighted.)?

Scenario:: UFO's (or flying saucers,if you prefer that name) are seen in the skies Some people take photo's or video what they see.Some witnesses seek a scientific esplanation, & so they approach a group of scientists & ask "What is This?", even handing over film & photo's, or saying "look,see for yourself, it's there." (pointing at the object in question.) .... My Question is this:::: Before a scientist says Yes it is a spaceship or No it must be something else,, HOW Does the scientist reach that conclusion? What is the logical process of evaluation to reach their conclusion?