Years ago when items that were sent in the hands of goyim were not professionally sealed, there were very specific signs that were made on the packaging to assure that it was not opened by the goy. But, nowadays where we have large kashrus organizations approving items that are packaged and sealed in many different ways, we are compelled to analyze what type of seal and packaging is sufficient to remove the concern that the contents weren't exchanged.
First, it is important to note that all these rules are only applicable if the goy would have some motivation to switch the kosher item for a non-kosher item, as Tosafos 12a (and 32a) writes. Generally, a goy would have no motivation to swap one item for another since the kosher item isn't better or more valuable to him that the treif one. Secondly, the professional packaging may in and of itself qualify as a double seal since it is almost impossible for an individual to duplicate a factory sealed item. Nevertheless, it is worthwhile to explore the halacha in this area.
In the Braisa 31a there is a machlokes whether items sent in the hands of a goy require a double seal (Rabbonon), or whether it is sufficient to just have one seal (R. Eliezer). The gemara paskens like R. Eliezer that a single seal is enough. But, Tosafos points out that Rav on 39a holds that for certain (expensive) items including meat and wine a double seal is necessary. How does this fit with R. Eliezer who permits a single seal on wine? Tosafos offers two approaches: 1. When sent in the hand of a mumar (which would probably include a jew who doesn't keep kosher) a double seal is required since he is self righteously assumes he will be trusted and is more likely to cheat. But when sent in the hand of a goy, one seal is sufficient (Tosafos rejects this approach). 2. When the seal will be checked in the end, one is sufficient, but if it won't be checked such as when sent from one person to another, it is necessary to have 2 seals.
Tosafos 32b confirms the approach that an item must either have 2 seals or be checked in the end, based on the braisa which says - אם מכיר חותמו וסתמו מותר. Rashi clearly understands that even according to R. Eliezer, it is necessary to check up on the seal and recognize that it wasn't tampered with. In the the retail market place when the sender doesn't check up on the seal, Tosafos holds that it should either require 2 seals, or a letter to the receiver describing the seal plus notifying the goy that the receiver will be checking up on the seal (it is clear from Tosafos that the letter doesn't replace recognizing the seal and only works because the goy is aware of the letter and scared to switch the item). However, Tosafos writes in the name of Rabbeinu Tam that according to R. Eliezer, the rationale to allow a single seal is that we aren't afraid that the goy will swap the item, so it isn't even necessary to check up on the seal. But, Tosafos only relies on this bidieved since the braisa does require recognizing the seal. At the very end of Tosafos they offer an alternate p'shat in the braisa that recognition of the seal is unnecessary. The requirement to recognize is only to the exclusion of one who finds the seal to be tampered with, but without any knowledge either way it is permitted - but concludes והמחמיר תבא עליו ברכה.
The discussion until now is when the item was received with at least one seal. However, the Rama (y.d. 130:8) is even matir when the seal is found to be broken, so long as it seems that it was broken accidentally and not tampered with. However, the Nodeh B'Yehuda (pischei teshuva 2) limits the Rama to a case where the goy isn't aware that the seal was broken, but if he was aware that it was broken, albeit accidentally, the wine or meat remained in his hands without a seal. Based on this, if one would purchase a kosher product (i.e. meat or cheese) in a store and the package was opened, it would still be permissible because we can assume that the goyim who work in the store aren't aware of the broken seal. But, if they would notify you of the broken seal, even if they would say that it opened when it fell, it would be forbidden to use the product.