I've got this great poem, or song. Maybe a recipe or a pedagological work. Is it period? How do I know?
Excellent question! I'll tell you the truth. I don't know. Even under the best circumstances there is room for uncertainty.
I found it in a Period manuscript.
Great! An excellent start!
Start?
Yes. "Start."
There are few pitfalls to watch out for. First I'll mention is the Medieval practice of palimpest. Parchment and vellum were expensive. Writings thought "unimportant" would be erased, the parchment used to create a new document. It might be possible to carbon-date the substrate parchment when the actual writing is hundreds of years newer. Palaeography, the study of old writing, can be helpful in these cases.
While it would be rare for ancient parchment to be re-used far past Period, a number of forgeries have occurred. Buy your favorite research librarian a bottle of wine; they deserve it.
There are modern techniques involving x-rays and other methods that can nondestructively determine the chemical composition of inks and curing compounds. The study of books as physical objects is codicology. This topic has occupied entire academic careers.
Someone told me to read a book, but the book is Post Period. Should I be reading it?
Consider Palistrina, From his own time in the 15th Century well into the 19th he was regarded as the greatest of Renaissance composers, spoken of in a single breath with Bach, Mozart, and Beethoven. Until the 20th Century the definitive book on Palistrina's style was Johann Joseph Fux 1735 Gradus ad Parnassum. If you ask me to teach you Counterpoint, I will print you off a copy and tell you to work through all the exercises. All of them. It's that important
So this story was collected by Grimm. Is it Period?
Good question! Look at Cinderalla. The Grimm brothers worked in the 19th Century, their tales promising bloody retribution against Cinderella's tormentors. More familiar to Americans is Disney's version of Cinderella, based on an English translation of Charles Perrault's collection of 1697. But the oldest recorded version is the tale itself Ye Xian, recorded in 860, and seems to have arrived in Europe via the Arabian Nights. This means that even if you rely on Bruno Bettelheim's The Uses of Enchantment or Anne Sexton's Transformations as your source, you're still telling an arguably Period story. For an A&S competition I might suggest looking at older sources, but I can't fault your story archetype.
What about expressions or turns of phrase?
Prior to the 1820s, a gown might have a train, a king might have a train of retainers, and you might speak idiomatically of your train of thought. An army had a baggage train. When railroads were invented, locomotives hauled "brigades of cars" for several decades. But when Virgil Caine "rode on the Danville train," the context makes it pretty clear he's talking about American railroads, even without the next line about the "Winter of '65"
An astounding number of common English expressions were coined by Shakespeare or by the translators of the King James Bible. In English, the Oxford English Dictionary can be useful in determining when a word or phrase was first used in a particular way.
What about people?
I sing The Ballad of Captain Kidd. Captain Kidd refers to William Kidd, executed in 1701 for piracy. Clearly post-period.
But suppose you find a song that refers to "Charles." Is that Charles I of England (Post Period) or Charles Martel (Period)?
There's also topical variation. A song lauding a king might be updated with the name of the latest king, or a song decrying an enemy king being updated for the latest enemy.
Place names?
The song Donkey Riding (19th C) refers to Quebec (founded in 1608) as a common merchant port, and to "rounding cape horn" as a routine, if brutal, journey. Europeans didn't navigate Cape Horn until 1618.
This can work in other ways as well. The protagonist of Whiskey in the Jar (18th C) is variously from Ireland, Scotland, or Kentucky. The provenance of a particular set of lyrics can be difficult to trace.
It refers to anachronistic technology.
Something like Chemical Worker's Lament is obviously tied to the 19th and 20th Century labor movement, clearly of its time.
But consider Donkey Riding again. I first heard the song in a TV ad for the Girl Scouts, where they sang the song while riding on the shoulders of a couple of older girls wearing a donkey costume. I didn't know about Quebec or Cape Horn yet, but a donkey as a dockside dray animal it made perfect sense.
But "donkey" can also refer to a small steam engine used to run winches around logging, mining, and shipping facilities. The ones used for logging were often mounted on a sledge so they could draw themselves to the next place they were needed. This makes sense given the fairly straightforward origin of the text, 1840s Newfoundland where that sort of "donkey" was common.
River Driver's Lament is an interesting case. You have to know quite a bit about the history of the timber industry to know that mass log floats didn't occur until the circular saw, which its much faster operation, made them economically viable. Most SCA song circles would sing this with out a second thought.
So I shouldn't sing Erie Canal or recite Robert Service
Different question entirely. If you're entering an A&S competition, Robert Service probably isn't your best choice, no. But are you busking? Bardic circle? Around the fire in your camp? Get all the fighters chanting "It's the MUD! MUD! MUD!" at a muddy muddy Pennsic and they'll remember you forever. If your camp appreciates Erie Canal, you might reconsider your drainage arrangements
This is a complicated topic. There are rarely straightforward and simple answers that will satisfy everyone. One piece of advice I borrow from Chairman Kaga on the original Japanese Iron Chef. Everything he said was prefaced with "If my memory serves me..." because to be wrong is to lose face, a big deal in Japenses culture.
Don't be afraid to express your uncertainty. I can't learn anything at all if I already know everything. Now if you'll excuse me, I have a request for Baby You Can Drive My Car.