Some signs a shy guy is interested in a woman include looking at her when she's not looking, trying to make small talk and sharing personal information. His body language seems uncertain. He fidgets and has difficulty making eye contact. He may also show irritation when the object of his affection flirts with other guys.
A shy guy who is interested in a woman shows interest in her hobbies. He may suddenly take up the same hobby to spend time with her. He remembers small details of conversations that other friends don't.
Shy men are often more confident communicating by text or online than in person or on the phone. He may fade into the background at large social gatherings, but find a reason to message her later. Sometimes, a shy man confides in his friends, and a woman may notice that they tease him when she's around, or they try to set up situations where the two are alone together.
A shy guy may seem interested, but never ask the woman out because he fears rejection. Make the first move, and a shy guy may open up. Alternatively, he may find the confidence to come on strong, but retreat if he's not met with absolute enthusiasm.