Most applications/websites either have an icon (often a question mark) to indicate you can hover over it (but then the hover only works on the icon, not the label) or no special formatting at all. The answer here suggests using a dotted line as well; I vaguely remember old Windows (3.x) help files working the same way.
Stack Exchange has a lot of labels / links with tooltips:
- almost all dates/timestamps (the 'asked 14 mins ago', which appears on your user card while I'm writing this, the two 'today's currently under the question title)
- the links under each post ('share', 'edit', 'follow', etc.)
- most cases where a number larger than 1000 isn't shown exactly, such as the number of views of a question
They do not have a common style. Perhaps if they would, users on a touchscreen device would wonder about what the common style would mean; after all, they are unable to access hovers.