1. Is the post out of character for the person posting it?
I'm going to go out on a limb and say that grandmothers typically have no desire to send porn links to their closest friends and family members. This post is obviously way out of character for her, making it highly likely that someone else is using her account. The link that was sent from her account will likely send you to a phishing site or prompt you to install a rogue Facebook application that might pillage your personal information.
2. Is the the language used in the post normal for the region?
My grandmother was many things, but a bad speller with a poor command of the English language wasn't one of them. Given the global reach of the Internet, a Facebook account can be hacked from anywhere in the world. Hackers will try to impersonate the user of the hacked account as best they can. The problem is that if the hacker isn't a native of the country of their victim, then they might be unable to accurately imitate the colloquial expressions or local slang used in the country of their victim's hacked account.
Let's imagine an example:
Actual post from grandma: "Good luck with your tests next week honey. I'm sure they will be a piece of cake."
Hacker trying to impersonate grandma from her hacked account: "May luck be with you for your examinations. It will be a piece of pizza."
This should be a dead giveaway that grandma's account was hacked or at least that someone needs to check and make sure she's taking all her proper medications.
3. Does the post ask for money or phish for personal information?
There is a popular scam on Facebook where a hacker using a compromised account impersonates someone and posts that they need money because they are lost in a foreign country or stuck somewhere without their passport, wallet, etc. Eager to help a friend in need, their Facebook friend wires them money only to find out later that they were duped by a hacker.
What if your friend is really lost and in need? You would hate to leave them stranded, right? Call your friend or check with their family to see if the story holds true. If you can't verify the story by phone or other means, ask your friend (or the hacker) questions that only your friend should know the answers to (and not stuff that they could find on your Facebook profile page).
4. Does the link in the post look strange or use a link shortening service like Bit.ly?
People love using link shortening services because they can take a huge web address and shorten it to just a few characters, making it easier to remember and short enough to fit into a Twitter post. The problem is that link shortening services like Bit.ly are often used by hackers to mask the true destination web addresses of phishing sites or other harmful web-based malware.
To verify the true destination of a shortened link you should check the link with a link lengthening site like Untiny. The lengthening site will show you the destination redirect link without having to visit it yourself. This lets you check to see if it's safe before going there.
5. Was the post placed on the walls of all of the poster's Facebook friends?
If you see an odd post on your wall, check to see if it's also on the walls of some of your mutual friends.
Many hackers and rogue Facebook apps will attempt to spread their links like a virus by abusing the "Allow Friends to Post on My Wall" Facebook permission that many of us have enabled. The hacker and/or rogue app will usually post the same scam or phishing link to the wall of every friend on the victim's friends list. This allows them to rapidly spread the link or app to as many people as possible. Additionally, the posts are spread even further because wall posts show up in the live feed and many people will share something without even visiting it first.