That's funny 'cuz I've been there about 20-25 years ago when I was in college
You have to look at the front car on the train when it comes in, that will tell you where it's going. Alewife is the last stop on all northbound Redline trains I think. If that lady put you on a train to Arlington station then you were on the green line. That's the most confusing of all the T lines.
Getting to Park St. will get you anywhere you wanna go, you just have to be able to read the maps.
Going out and back into the station is definately gonna cost you. Every station I've been at you can you can turn around and go back to the other side of the tracks and head back where you came from without going through a turnstile.