In large dictionaries like 広辞苑 or 大辞林, there are some entries tagged with "ancient pronunciations" which are almost identical to the modern ones but with a voiced consonant voiceless (or vice versa), e.g.
さわ・ぐ 【騒ぐ】〔上代は「さわく」と清音。〕pronounced voicelessly as "sawaku" in the early era
しろ-がね 【銀】〔古くは「しろかね」とも。〕also "shirokane" in ancient times
As I know, in most ancient texts, voiced syllables are written without any voicing mark(濁点), so they are identical to the voiceless ones orthographically. How do dictionary editors know that the syllables are with voiceless consonants?