[x] How can I find out if the accession of a particular State to a certain Convention has been accepted by other Contracting States?

For some Conventions, the accession only has effect in the relations between the acceding State and those Contracting States which have accepted the accession.
First click on "Conventions" in the left-hand navigation menu and select the Convention you are interested in. Then press the button "Status table" in the right-hand navigation menu.
Where you see A* in the column "Type", the accession of the corresponding State has been accepted by other States. Clicking "A*" will lead you to the table of acceptances of the accession. This table shows which Contracting States have accepted the accession, the dates of the acceptances and the dates of entry into force between the acceding State and the accepting States.