Lost in the rush to get everyone masked up during the pandemic, was the importance of having a mask that fits properly! Any old mask can be more or less effective at blocking sneezes and large water droplets on your breath, but ineffective and improperly sized masks (including N95 masks) allow quite a bit of air flow around the edges of the mask.
It goes without saying that even the best masks can only filter air that passes through the filter medium and fail to provide any protection from air that enters through gaps between your face and the mask.
Some medical facilities, labs or manufacturing operations may use a testing system to test the fit of users' masks, but that's pretty impractical for most of us.
Do you know your mask size? It's determined by measuring the distance between the cheekbones and the distance from the bridge of your nose to the tip of your chin. Here's a handy guide you might find helpful: