Checking the cap seal with flexible plastic bottles is easy.
I pull the foil seal and remove ALL the seal.
When I install the cap I squeeze the bottle and listen for air escaping.
If I hear no air escaping I have a good seal.
If I do hear air escaping I try to figure it out, it is usually a little debris under the cap.
If I can't seal it I don't keep that particular bottle. Only once has this happened.