There are several ways to tell if a vinyl floor has been properly sealed. One way is to check the seams of the flooring, which should be tight and without any gaps. Another way is to use a black light, which will show areas that have not been adequately sealed. Another way to tell if a vinyl floor has been properly sealed is by checking for bubbles in the seams of the flooring. If there are bubbles in the seams, there was not enough sealant applied during installation, and it may not be possible for these areas to be resealed successfully.