It seems to me that there should be a Latin phrase that is used in English that basically means, “consider the source”.
Person A: Oh, the dealer from brand X told me that brand Y sucks!
Person B: Consider the source. (I am looking for a phrase that would substitute for this)
Similar to phrases like caveat emptor and cui bono.
When I do a search, all I get is literal translations, usually with several options for the words “consider” and “source”. Does anyone know if there is such a phrase?
Sounds like the closest thing to what you want might be the Latin proverb Testis in uno falsus, in nullo fidem meretur or Falsus in uno, falsus in omnibus: “A witness false in one thing deserves trust in nothing”, “False in one thing, false in all”.
Other possibilities:
[ul]
[li]Ex uno disce omnes - From one example you may deduce the rest.[/li][li]Ecce signum - Literally “behold the sign”, but a freer translation would be “examine the evidence.” Of course, if you’re punning, “behold the sign” might stretch to refer to, say, a brand name. Maybe?[/li][/ul]