Twitter have been pretty comprehensive in stopping you from seeing who's blocked you.
As the answer above mentions, there are still a few ways to see if you've been blocked, but honestly - if you suspect that someone has blocked you on Twitter, I'd recommend just blocking them back and moving on with life.
If it really matters to you that much, just ask whether they saw your last tweet.
if they act suspicious, then you might want to ask if they've blocked you or not.
Either way though, there are plenty of cool people on Twitter, no need to worry about one user out a several millions.