Countdown timers and low-stock warnings are some of the most common tools used to push a purchase decision faster than you'd otherwise make it, and a good chunk of them are not tied to anything real.
A simple test: reload the page after clearing your browser cache, or open the same product in a private/incognito window. If the countdown timer restarts from the same amount of time, it isn't counting down to an actual event, it's just a script that always shows a fixed window from whenever you loaded the page.
'Only 3 left in stock' warnings are trickier to verify, but a useful signal is whether the number changes at all if you check back a few hours later without buying. If it's still '3 left' the next morning, that number probably isn't tracking real inventory.
None of this means every urgency signal is fake, some flash sales are genuinely time-boxed by the retailer's own promotional calendar. The difference is that real ones tend to be announced in advance (an email, a banner days earlier) rather than appearing to pressure a single visit.
The practical habit: if a deal seems worth buying, it's usually still worth it tomorrow. Fake urgency relies on you not checking back.