How would you know if you woke up 20 years in the past?

I'm writing a short story in which the main character gets sent back in time by nearly 20 years, but doesn't know it. Essentially they wake up in the past, a couple of years before they were born. The problem is that I'm having a hard time thinking of a way to get my character to realize that they aren't in the right time period. Is there some reasonable way to get my character to know they are in the wrong time? Having another character tell her seems silly. Besides who would believe someone who told them they traveled back in time?

Some things about the "universe" my character lives in. My character lives in a time in a more distant past, so there isn't much technological growth in the twenty years she travels. Its not like she wakes up and her IPhone 5 doesn't work. She lives in a world where magic is possible.

I think the first thing I would notice is the lack of iphones and ipads… Or any phones for that matter. The lack of headphones. And the lack of Wi-Fi. In fact, very few homes have the internet if any at all.

Lack of CD's and DVD's haven't been invented yet. Instead we have these great big things called "Records" and "Video cassettes".

This is up to you to think about. If I was writing this, I would be fun trying to think of ways the character could find out. Do you enjoy writing or not?

Well, I was going to suggest checking the iPhone when I saw your question first, but then I read the rest. Well, maybe they could meet someone they know, looking really young. Or what if they wake up in a church, and look to see their parents getting married. And there's always going to be some kind of thing that used to be trending years ago, but now, people just go like, "That's so 2012." For example, suppose she's in the Harry Potter world, she sees posters like, 'FIRST EVER QUIDDITCH MATCH IN THE UK". But when she thinks back, she realizes that the first match in UK was 20 years ago… Get creative, it doesn't have to be something big. Make it in really small details that seem unsettling to the character at first before she realizes that she's in the wrong time period.

Surely the house across the road would look newer, her parents or neighbors would look younger, maybe current events would be a clue for her, or people giving her strange looks because she's "new around town", generally though, fashion is one of the quickest changing things (besides technology, but you said their isn't much growth), so she may be wearing something differently, or wearing different things all together.

But like David said, this seems like it would be one of the most fun parts of writing that - thinking about how to bring the character to the realization that they've woken up 20 years in the past is a gold mine for creative ideas.

Music has changed

They should first start seeing some buildings that look really familiar as they walk down the street, puzzled as to where they are. Then, walking faster, not knowing where they're going, and suddenly they recognize the street; it's the one they grew up on! They stop in front of the house one of the main character's mother and father lived in since their marriage date, which was about, let's say, five years before he/she was born. They walk up to it, and knock on the door. When his/her father or mother answers, they look much younger! *insert drama here* On impulse, he/she asks if their son/daughter is home. Then, the parents are all like, "Wrong address, we don't have a kid." They ask what year it is, and then realize it's 20 years previously! *GASP*
That's all I got XD I also think they should mess up some stuff in the future, then wait 10 years and be like, "OH we can fix this!" and have to go back and find themselves after they get sent back to tell them not to make that mistake and guide them back to the after 20 years, drop them off, and then return to the 30 years. But you might not wanna do that XD I just think it'd be cool Lol XD

As they approach their younger selves, their memories begin to return.

The easiest way to tell is the automobiles. There would be none that were made in the last 20 years and it would be obvious.

20 years ago, cell phones were new and expensive and about the size of a brick. You had to use a hissing loud modem to connect to the Internet. A lot of sites for checking up on someone were free, now you have to sign up and pay.