My 99 Forester is showing code: PO440. This is a vacume leak. Any suggestions what I should check first? What usually causes this?? How do you check a gas cap?

Do a search in this form for the code. Lots of post out there.

make sure the cap is tight aleast 4-5 clicks , if it's a vacuum pop hood listern and look for loose hose ,spray carb cleaner around all vacuum lines and see if it affects anything

My 99 Forester is showing code: PO440. [...]

First take a look at http://endwrench.com/images/pdfs/Foresterinfo.pdf

 

If that's not the cause, then a common problem is rust through of the gas tank fuel filler pipe.

It can be a rather small hole.

 

If not that, see:

http://www.endwrench.com/current/02SpringInsiderInfo.pdf

http://endwrench.com/pdf/fuel/MoreP0440TipsInfoW00.pdf