Shy guy or not interested how to tell for sure?

Calling all guys!!! Is there any sure fire way to test a guy to see if he acts a certain way around a girl b/c he is shy or not interested? How can you tell if avoiding eye contact or avoiding the girl altogther or being nervous and uncomfortable around a girl is b/c he is shy or just not interested?

In my situation he doesn't look at me when speaking to me at all
-I wore make up for the first time and he noticed and told me that I looked good. This rised my level of interest in him and it made me feel great
-We don`t always work together so I try to bump at him at work often and when I see him I freeze up since I am shy so I just say hi and all I get is hi back not much else

-yesterday we worked opposite shift so when I arrived on the floor, he was leaving. There were also few co workers with me. He didn't even look at me at all and didn't say hi or anything

He is just not interested?
Shy guy or not interested how to tell for sure?
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