There are a number of factors to be considered when evaluating a conference to determine if it is legitimate or predatory. Presence or absence of one indicator does not determine a conference's legitimacy. The cumulative effect of indicators is what matters.
Indicators of a Legitimate Conference
The conference website if free from spelling or grammatical errors
The conference website has full contact details for questions
The conference is sponsored by/in partnership with a reputable, established group
The conference program is available
Conference proceedings are published/indexes/made available after the event
Keynote speakers are recognized experts in the field
Expectations and instructions for authors regarding submissions are clearly stated on the website
Submissions for papers/presentations/posters undergo a review process for acceptance
Indicators of a Potential Predatory Conference
Pressure to register quickly to "reserve" a spot
Registration is required to obtain information about the conference
The stated purpose of the conference is vague
Conference organizers offer multiple conferences at many locations at the same time
Conference organizer sends unsolicited email invitations to new conferences.