No, they can't determine if it's been read even though the posts sometimes contain code that tries to track it.
It's a bit more like nuisance phone calls (which are dialled by robot) - once you've answered the phone once, your number is logged on a database as 'real' and then gets sold on to the next spamming company.

Obviously if I'm reading the bb then I'm going to see the message in question before I see any replies to it anyway.
If I miss something, alert me by hitting the trouble ticket link at the top as I will see that even if I am out.

Might surprise people to know that about 10,000 attempts to post per day do get blocked but the odd one gets through probably because it doesn't contain any phrases that are out of the ordinary.