Hello everyone,
- An italic font is a specially created font and apart from the rightward slant, some glyphs have a different design to their Roman equivalents, for example a and g.
- An oblique font is a Roman font slanted to the right.
- A slanted font could be slanted to the left (but Amiri slanted is slanted to the right so a priori either italic or oblique, but many characters are identical to the regular form!)
Finally, how can one really be sure that a font named italic or oblique or slanted is so?
Thank you for your explanations.
Look at it ! If it looks slanted or italic then it probably is. 
See Font Properties, Masters, General to see whether the font is Regular, Bold, Italic, or Bold Italic.
When a user selects the italic attribute in a word-processor, the italic version of the font is used.
Italic fonts usually have a negative italic angle, but they do not have to actually be slanted.
Script fonts may be Regular type style, but still slanted, e.g. Hari Regular.
Properties Master displays what has been entered into the font by the author.
Here, the master is (mischievously) called Casimir for a Google Roboto Regular font registered as… Roboto Bold.
How can we be sure that the Italic, Slanted, Oblique information the author has entered is correct?
