How can the police determine which way a window was broken in?

If the glass is still there, it's what side of the window it's on.

I agree, in most shows I've seen they always refer to which way the broken glass lies.

What the others said: if the glass is on the inside, the blow that broke it was from the outside, pushing in. If the glass is lying on the outside, the blow was from the inside, pushing out. So, if someone is trying to fake a burglary, and stands in the kitchen and knocks out a glass pane on the door, the glass lying outside the door is a giveaway that the 'burglar' was not outside, breaking in.
(If it's a really big window, a little glass might fall back into the room, but this wouldn't equal the amount being pushed out.)

ironpony said:

Okay thanks. It's so obvious now and I didn't think of it that way lol. I thought maybe if you hit the window, it could likely shatter with glass on both sides. But yes the character is trying to fake a burglary so I can write that way. Thanks.

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What do you want the audience to think about the character? Because smashing the window from the wrong side is a fairly basic error.

This is true. I'm no criminal, nor have I have locked the keys on the wrong side of the door. I learned this stuff watching 'Forensic Files' and other true crime on TV. (A goldmine of information, turns out law enforcement people like to describe how they caught people, even if it takes them years.)

Actually you can tell by the way the glass shatters. There are lots of sites you can Google which explain how glass behaves when impacted by a blow.