You know you are in the blind spot of a truck in front of you if:

When you can't see the truck driver in their mirror, it's a clear indication that you are in their blind spot. Trucks have large blind spots on both sides, directly behind them, and sometimes right in front. If the driver is unable to see you in their mirror, they may not be aware of your presence, which increases the risk of an accident if they decide to change lanes or make a turn.

Being able to see the driver in their mirror indicates that you are in a safe position relative to the truck. Hearing the truck's engine does not provide any information about your position in relation to the driver's visibility. Similarly, tailgating the truck decreases safety but does not define if you are in a blind spot; it simply indicates that you are too close, regardless of visible sightlines. Thus, the correct understanding of being in a blind spot is directly linked to whether or not the driver can see you in their mirrors.