In the US at least there is the site listing canonical bishops. And you could always verify the bishop, and lineage elsewhere.
Our parish's website lists all that info pretty close to the front, easy to find. Not harping on it, but who is our bishop and what the line of jurisdiction (?) is. If someone doesn't want to share that info, I'd say it's a red flag. But often schismatics do, and then it's a matter of tracking down whether their hierarchs are canonical.
Sometimes the language is a giveaway. The Real True Honest to Goodness Only Orthodox Church on the Planet More Orthodox than Anybody Else (joking - but only partially) ... or comments about how their bishop has been "persecuted" by the Church (likewise even personal comments alleging bad treatment from the hierarchs ... I've met a few schismatics who spoke very frequently of being supposedly mistreated).
I suppose it depends on the reason they schismed. But I see those particular things often and at least in the most dangerous schismatics.
(I think a lady who goes to our parish might be actually from and prefer a particular schismatic group, but it's only a fairly minor disagreement in her point of view - the calendar - and both calendars are legitimately Orthodox anyway - and she's very pious and seems to be nothing wrong with her theology or practice.)