The difference between a guy that has a genuine interest in you and one who doesn't, is that the interested guy will give you a specific explanation about why he behaved the way he did, while the disinterested guy will give you a vague explanation - or no explanation. (An overly-interested guy will give a rambling and specific explanation with apologies interlaced between the details - but you aren't worried about how to spot men like this, you are more interested in losing them.)
Take the first example - your guy goes to a bar with his coworkers without telling you about it. On the one hand he might be out trying to pick up some new women, but on the other hand his phone might have legitimately died so he may have had no way to contact you.
Consider the range of explanations he could give you:
The Overly-Interested Man
"Babe, the company decided at last minute yesterday to have a going away party for Joe, and I'd forgotten to charge my phone last night so it was completely dead by the time we left for the bar. I had no way of telling you where I was; I'm sorry. I had to go to the party, too, because my manager was going to the party and told us all that he expected us to be there. Babe, I'm sorry, I would have called if I'd been able. I even thought about using Neil's phone to call you but I couldn't get your number off my phone. I hope you don't mind."
The overly interested guy would continue to give obsequious explanations if you pushed him on it - but you probably wouldn't since you'd already trust him and not care much about where he was anyway.The Interested Man
"Sorry about last night. I wanted to call you to give you a heads up but my phone was dead and my manager insisted that we all go to this impromptu going away party for Joe. It was at Bootleggers, downtown - cool place actually. We should go there next time we go out."
The interested guy would follow up his explanation with a little more detail if you pushed him on it, saying something like "Yeah I know it was dumb not to charge my phone, but then I was in that meeting I told you about all day today, so I didn't realize until we left for the bar that it was dead." or "No, I know it was weird, but I think my manager realized that Joe might be working for our clients in the future, so he wanted to send him off on good terms - hence the need for a good turnout."The Disinterested Man
"I went to this thing for work."
Typically if a man is disinterested in a woman, she will be interested in him - so she won't push for more details because she'll be worried about upsetting the already-precarious relationship. But if she does, she is likely to be met with something like "It was a party for a guy who is leaving" or "Oh, it was nothing; don't worry about it."This whole phenomenon boils down to the fact that, in proportion to his interest in you, he will be concerned with your opinion of him, and will want you to understand why he behaved strangely.
It is worth noting that some men might need you to express your disapproval before offering an explanation for their behavior - men are sometimes oblivious to a woman's feelings. But if you express your disapproval or tell them (calmly and reasonably) that you were worried or upset, this should elicit a specific explanation for his behavior, not a vague one.
Finally, while it would be tough or even impossible to perfectly correlate his interest to the degree of specificity in his explanations, you can still use this principle by bearing in mind this range of possible explanation types, while asking yourself "Is this how a man who cared about my opinion would explain himself?"
1. The More Confident He Is, The Less He Likes You
2. How to Deal with Men Who Have Bad "Game"
3. "Because Of" Versus "In Spite Of"