Knowing - I think the only way to know for sure if someone is 100% single is to have gone back to theirs and spent time with them to make sure they're not seeing someone else. Knowing they're not married is easier, as presumably they'd be living with their OH.
I wouldn't say that him saying he needs his space on the weekend is an amber flag per se - but him saying he didn't get in touch so as to give you a break is a bit amberish. In a 'I did something you wouldn't like, so I'm going to blame you for it' type of way.
I dated the Artist this time last year for about 2 weeks, following emails online for a week and a bit, I think. She texted me every night, she called me for hours, I at first responded enthusiastically and then to be honest out of politeness. She'd text me from work and get annoyed if I didn't answer within a couple of minutes, she wanted to skype me so she could 'meet' my kids, and wanted to meet them in person. She was, in the end, unnervingly intense.
I wanted someone who would respond to my texts when I was in the mood to chat, occasionally start a conversation herself but know instinctively when I wasn't in a chatty mood, and leave me alone when I was with my DC. I know this is ridiculous and unrealistic and a bit selfish, but I live in hope that I'll meet someone who won't take offence when I tell them to cool it a bit, and know I'm not trying to break things off, just to go at my own pace. People have different rhythms at different times, and finding someone who matches those rhythms and who matches yours is a tough job, not necessarily an amber or red flag.
Some women love texting, some men do, some don't. But as with any OD related thing, don't read too much into it. Wait for him to initiate the evening texts if he's feeling put upon by the intensity so far.
I'd also meet sooner rather than later because it is, as we say, all bullshit until it happens. If he's not willing to ask you out sooner, drop into conversation that someone else wants to meet, and you'd like to see if he's ever going to bother asking you for a drink. If he likes you, he'll ask. If he doesn't, drop him.
Fail fast, fail cheap.