1. Presbyterians appear to have no calendar other than the one you buy at a thrift store. Minimal holidays, no guidance on Bible reading. This can be disorienting for those of us who need the Bible to get through the day. Suggestion: buy a King James Bible. Yeah, the adult bible, the one you need a dictionary with. The King James Bible Companion is an ideal bible dictionary: it's cheap (50 cents), done by an expert, and only 24 pages. Relax, the KJV is easier than the Shakespeare you had to read in 8th grade. Now, add to it a year (date) reading plan typically on a card (or a page, see this (PDF), and have a couple of other friends track along with you. You can share insights and questions at lunch on Sunday. Don't worry about the other bibles printing something else: they have to have their own copyrights to make money and want to be paid for their work. Most of the modern bibles don't even use the text the KJV was using, but a compilation with a recent copyright date to translate from. The KJV is based on the same text John Chrysostom was using to explain the scriptures (347-407), as well as the one translated into Gothic for Western Europe by Ulfilas (311-383). This is the text that the Church has carried through history, the one that is the majority of old manuscripts we have.
2. Watch for the pastor to worry about sins that aren't notably present in his congregation. This goes further than being terrified of preaching 1 Peter 3. If you're attacking the sins of people not there, don't be surprised if the sins of people there increase.
3. Presbyterians tend to insist on the latest 'easy-to-read for children' bibles. They disdain the KJV, saying they can't understand it. These people will usually admit to enjoying various Shakespeare plays if you bring the conversation around that way, so they are not being truthful, since the KJV is much easier than Shakespeare, even in his easy plays (like Romeo and Juliet). The oddest thing is that they use these children's bibles in the adult service, trying to wrest them into saying the words they need them to say for the next point in the sermon. Don't be surprised in the midst of this "easy to understand" bible reading they interject Greek words, or Hebrew or Aramaic words, which the congregation doesn't understand, but can repeat the sounds of. They don't really want an easy to read bible. They want difficulty, obscurity, and elite password status.
4. Presbyterians don't always want to believe what's in front of their eyes. Watch for them to bring up both Greek Bibles copyrighted a handful of years ago and a short lexicon whenever they disagree with the bible they are reading. Lexicons are a poor tool since they have no feeling for the structure of the verses around which the translation is occurring, and therefore advance suggestions that a reasonable person would flee from. But they are used because they supply many words to allow for many interpretations of a word in Greek to English. They are meant for experts in Greek as a short reference. If you're using the KJV and the Companion recommended above, see what parallel constructions you can find near the word you don't understand, or use Cruden's Concordance to find several uses of the word.
5. It is accidentally funny to note Presbyterians' use of Greek words like "agape" and "phileo". They don't know they are translating from the Koine trade language rather than the high Greek language. The words are synonyms in Koine. They don't know any better. You may, if you find open-minded ones, offer them a test on which is appropriate to the verse. Watch them do slightly better than coin-tossing at getting the words correct. Sometimes they take the point. If you want to see this in action, watch Sam Gipp on Youtube: search for "Sam Gipp phileo" and spend an enjoyable hour. The real contrast in English is between "charity" and other relations. Charity is much more important (and means more than throwing pennies to orphans: take a moment with an OED and learn the word thoroughly).
6. It is not so funny to notice Presbyterians using words like "Yahweh" for "Jehovah" despite the thousand-odd ancient Hebrew manuscripts which use "Jehovah," and the many uses of "Jehu" or "Jeho" as part of a name, indicating which word it was derived from. It is definitely not funny to hear them use the word "Shekhinah," which is the name of a Canaanite goddess, consort to Yahweh, a Canaanite god. The word occurs nowhere in the Bible, and is an example of why we don't use the Talmud to understand the Bible.
7.How much time do Presbyterians spend urging people to do the deeds of the law? Lots. Some of them even use the phrase "we are justified by our actions," even though St. Paul disagrees with them (Rom. 3:20). Most are ignorant of the fact that it is God who transforms us (See Phil. 2:13), not we who earn it. Our role as toddlers is to pray always (1 Thessalonians 5:17), read his Word (Ephesians 6:17), walk where God shows us (Eph. 2:10), keeping in the will of God e.g. here, and confess to him our failures (1 John 1:9). Community projects are invitations to "walk" (in our case, toddle), in good works. We don't know what we accomplish.
8.A surprising number of Presbyterian churches have people who insist that they possess the charismatic gifts. Start with the assumption that they are deluded, and see if they can be avoided without surprising people. If not, find another church. Yes, God can do anything, no, I don't know his plan, but I note that the Bible is complete.
9.Poor Bible translation has consequences: Rom. 16:1 describes Phebe as a "servant", because, as we know from 1 Tim. 3:8,12, deacons are men, and we have no record in Acts of a church appointing a woman to this place. But once you change it to "deacon" or "deaconess", then you have an excuse to nominate women to church offices, and few churches can resist. Suddenly, they don't want to pay attention to the Bible speaking clearly against their next step (appointing women as teachers and preachers: 1 Cor. 14:34-35), because it doesn't fit what they want to do, and who they want to think of themselves as.
10.Hallmark holidays will show up, typically with sermons that make you wish you had gone out for brunch instead. Mother's day (praising mothers), Father's day (condemning fathers), &c &c.