The relevant number is probably not D (the fraction of people who suffer from delusions) but a smaller number D0 (the fraction of people who suffer from this particular kind of delusion). In fact, not D0 but the probably-larger-in-this-context number D1 (the fraction of people in situations like yours before this happened who suffer from the particular delusion in question).

On the other hand, something like the original D is also relevant: the fraction of people-like-you whose reasoning processes are disturbed in a way that would make you unable to evaluate the available evidence (including, e.g., your knowledge of D1) correctly.

Aside from those quibbles, some other things you can do (mostly already mentioned by others here):

• Talk to other people whom you consider sane and sensible and intelligent.
• Check your reasoning carefully. Pay particular attention to points about which you feel strong emotions.
• Look for other signs of delusions.
• Apply something resembling scientific method: look for explicitly checkable things that should be true if B and false if not-B, and check them.
• Be aware that in the end one really can't reliably distinguish delusions from not-delusions from the inside.