How can you tell if you are getting the placebo or the real drug in a double-blind clinical study?

You can't- that information is supposed to be restricted such that not even the doctors treating you know which one it is (this is to avoid bias in the study results). When someone enrolls in a placebo-controlled trial, they will need to consent to this before enrolling, including acknowledging that they might receive a placebo. Keep in mind, though, that in most placebo trials, the placebo group is getting the best existing treatment plus the placebo, not placebo alone.

In emergencies it can be revealed if someone was on the drug or the placebo, and in rare cases where one group is obviously doing way better than the other, the trial may be unblinded, in which case everyone usually goes to the group that is working.

You should not be able to, or there is no point in the trial.

In cases where it is essential to know, for example when there is a serious medical problem or a possible drug reaction, the person can be withdrawn from the trial and told if they are taking the active drug or the placebo.

Wait until the study's over and the labels are broken. That's pretty well how these things are supposed to work, after all.