Check the indicator light. Like indoor vacuum cleaners, some leaf vacuums have a light that comes on when the bag is full.
Look at and feel the bag where the leaves are collected. When it looks and feels full, it probably is full.
Put your hand underneath the tube of the leaf vacuum but not actually on it. If it feel as if it is lacking suction, the problem may be a blocked filter, but lack of suction may also indicate the bag is full.
Note how well the leaf vacuum picks up leaves when it is empty. If it later becomes harder for it to suck up vegetation, the bag may need emptying.