When the bathhouse breaks into loud applause,
you will know that well-hung Mario is the cause.

[Audieris in quo, Flacce, balneo plausum,
Maronis illic esse mentulam scito.]

Martial (AD c.39-c.103) Spanish Roman poet, satirist, epigrammatist [Marcus Valerius Martialis]

Epigrams [Epigrammata], Book 9, epigram 33 (9.33) (AD 94) [tr. Juster (2016)]
    (Source)

(Source (Latin)). Alternate translations:

In whatever bath, Flaccus, you hear sounds resembling applause, know that there Maron's yard is to be found.
[tr. Bohn's Classical (1897); in earlier editions, just the Latin and an Italian translation were given.]

In the baths what is now the most pleasing diversion
Is to go and see Maro displaying his person.
[tr. Pott & Wright (1921), "The Wonder"]

If you’re passing the baths and you hear,
From within, an uproarious cheer,
You may safely conclude
Maron’s there, in the nude,
With that tool which has nowhere a peer.
[tr. Humphries (1963)]

It's easy to tell
by the roar of applause
in which of the baths
Maron is bathing.
[tr. Goertz (1971)]

If from the baths you hear a round of applause,
Maron's giant prick is bound to be the cause.
[tr. Michie (1972)]

When you hear applause in a bath, Flaccus, you may be sure that Maro's cock is there.
[tr. Shackleton Bailey (1993)]

The bath house applauds, with widening eyes,
When Maron reveals his astonishing size.
[tr. Ericsson (1995)]

The critics in the Baths rain bravos thick
For Marcus' coup de théâtre, his dick.
[tr. Wills (2007)]

If from the baths you hear a round of applause,
The giant prick of Maron is surely the cause.
[tr. Cooper]