I encountered this in Mozart KV 310:
Should I repeat the C
here?
The C should be repeated. In order to be considered a tie, the mark would have to begin at the first C and go directly to the second C. So in this case the mark is a slur.
Because the slur mark encompasses two notes (not just the C), then CE is played, followed by AC. A tie mark will always be starting and finishing at the same level, rather than slanting, as here. Simply because a tie will join two notes that are the same, but here, the mark is slanting, a good clue as to it being a slur.