how can I tell if somebody is lying to me?

Pay attention to the things they say at the moment and then pay attention to what they say later about it.

Sometimes it can be a gut feeling but even those are misinterpreted sometimes

Maybe they fidget in what they say.

Or, just Google it.

http://www.wikihow.com/Detect-Lies

http://www.realsimple.com/work-life/life-strategie...

http://www.psychologytoday.com/blog/ulterior-motiv...

There are certain physiological hints, such as pupils getting bigger when they lie. But these hints are highly inaccurate. So It would be unfair to call someone a lair when for example their pupils get bigger due to drinking alcohol.

So i suggest you just trust or not trust

try to make him understand that you realise his lies. never blame directly. and sometimes you can speak in an abstract way like "lying is something deplorable"...

nervousness - can't look you in the eye