Sealants are usually very white compared to the natural color of the tooth. Sealants also usually only fill in the grooves so it looks likes little lines on the occlusal of the tooth. Composites usually blend very well with the tooth but you can make out the prepped area and it covers a larger area of the tooth rather than the grooves. If you air dry the tooth you can also see the composite a little better.
I agree with WarnerSL. Also, check the radiographs. Sealants will not show up like a composite. The sealant will not be very deep at all. Appears very white and just laying on top of the tooth surface……fillings will always be much deeper on radiograph. Hope I’m explaining it okay…… :-/