You don't need a densitometer. The problem with the standard coin test is it doesn't account for the additive effect of safelight and enlarger exposure. The safelight exposure alone might not visibly fog the paper, but it might bring the paper to it's threshold exposure (the same way flashing does), in which case any further exposure under the enlarger, even very minimal exposure through dense highlights, produces tone on the paper. This decreases local contrast, particularly in the upper mid tones and highlights, and can lead to "muddy" looking highlights, even though the safelight didn't produce enough exposure to visibly fog the paper on it's own.
Originally Posted by cliveh