Maaser Sheni, Chapter 4, Mishnah 9. The Mishnah deals with the status of money that is found: "Kol hama'os hanimtza'im - harei elu chulin" - all coins that are found are chulin. One who finds coins need not be concerned that they are Maaser Sheni, and they are treated as ordinary money in every respect.
"Afilu dinar zahav im hakesef v'im hama'os" - even a gold dinar together with the silver and with the coins:
Even if a gold coin is found together with silver coins and copper coins, we are not concerned. At first glance one might have thought that precisely such a situation should raise a concern, since people do not normally mix coins of different types together, and such a mixture fits the pattern of Maaser Sheni: the redemption was made onto a large gold coin, and when there was change left over, additional coins of various values were added to it. Although there is an indication here, the Mishnah rules that we are not concerned, and their status is chulin.
The exception - Jerusalem at the time of a festival:
The only exception is Jerusalem at the time of a festival, that is, on the three festivals: Pesach, Shavuos and Succos. This ruling is based on the Mishnah in the seventh chapter of Maseches Shekalim, that in Jerusalem during the festivals one must be concerned that any money found is Maaser Sheni, since that was the season when the Jewish people brought their Maaser Sheni up to Jerusalem to eat it there. In any other place or time, these coins are chulin.
"Matza b'sochan cheres v'kasuv alav maaser" - if he found among them a shard with "maaser" written on it:
If a shard of earthenware was found together with the coins and the word "maaser" is written on it, it is maaser, and it has the status of Maaser Sheni. Even though only "maaser" was written, without specifying which maaser, we may assume that Maaser Sheni is meant, because Maaser Rishon, which is given to the Levi, has no halachic status beyond belonging to the Levi: it carries no restrictions, it need not be eaten in purity, and the like. Therefore an inscription of "maaser" indicates Maaser Sheni.
In summary: The Mishnah rules that all coins that are found are chulin, and there is no concern that they are Maaser Sheni, even when a gold dinar is found mixed with silver and copper coins, despite the indication suggested by such a mixture. The exception is Jerusalem at the time of a festival, when we are concerned for Maaser Sheni. And if a shard with "maaser" written on it is found together with the coins, they are Maaser Sheni, since only Maaser Sheni has a special status that calls for such a marking.