Shabbos (56a): But Samuel maintained: David did not pay heed to slander, [for] he saw self-evident things in him,22 For it is written, And Mephibosheth the son of Saul came down to meet the king; and he had neither dressed his feet, nor trimmed his beard, nor washed his clothes, etc.23
This gemora is used as the justification that if there is clear circumstantial evidence then it is not considered lashon harah to believe negative things about another.
Semag (Negative #10) :If a person sees in another aspects and circumstantial evidence which seems to validate the claim then it is proper to believe and accept that which is said as is stated in Shabbos (56b)…
Shulhan Arukh haRav(O.C. 156:10): One who accepts lashon harah is punished more than the one who says it unelss he sees clear cirucmstantial evidence.
However the Chofetz Chaim(Hilchos Lashon Hara, kelaI 7:10-11): takes a much more stringent view
חפץ חיים (הלכות אסורי לשון הרע - כלל ז:י-יב): י. וְאִם יֵשׁ עָלָיו (כב) דְּבָרִים הַנִּכָּרִים, שֶׁנִּרְאֶה עַל יְדֵי זֶה, שֶׁמַּה שֶּׁמְּסַפְּרִין עָלָיו הוּא אֱמֶת, דִּינָא הָכֵי (הַדִּין כָּךְ), אִם יֵשׁ בָּעִנְיָן הַזֶּה, אֲפִלּוּ אִם הַדָּבָר אֱמֶת, (כג) לְשָׁפְטוֹ לְצַד זְכוּת, אוֹ בְּעִנְיְנֵי שְׁלִילַת הַמַּעֲלוֹת, אוֹ בְּכָל שְׁאָר הַפְּרָטִים, הַמְבֹאָרִים לְעֵיל בְּסָעִיף ז', לֹא שַׁיָּךְ בָּזֶה דְּבָרִים הַנִּכָּרִים, דְּוַדַּאי אָנוּ מְחֻיָּבִין לְדוּנוֹ לְכַף זְכוּת (כד) כֵּיוָן שֶׁהוּא אִישׁ בֵּינוֹנִי כְּדֵי שֶׁלֹּא יִתְבַּזֶּה עַל יְדֵי זֶה בְּעֵינֵינוּ וְכַנַּ"ל, אֲבָל אִם הוּא דָּבָר אֲשֶׁר אֵין לִמְצֹא צַד זְכוּת עַל הָעוֹשְׁקוֹ, (כה) מֻתָּר לְהַאֲמִין וּלְקַבֵּל: