Is there any obvious sign that an R-12 A/C system has been converted to R-134?

I bought my 1984 Westy a few months ago without getting any records on it,
and the PO didn't seem to know much about it except the A/C "needed to be
recharged." Meaning, probably, that it also needs to have leaks repaired and
who knows what else.

Anyway, I wanted to fill it full of R-134 and see if I can get the
compressor to kick on, etc., but am not sure whether the system's been
converted or not.

No stickers saying one way or the other.

Ideas? Thanks.

Geza