Which countries have signed the Convention?

U can see which countries have signed The Convention and the date on which they did on the website of the HCCH :
https://www.hcch.net/en/instruments/conventions/status-table/?cid=24
The HCCH made a spreadsheet showing acceptances of accessions to the Child Abduction Convention:
https://assets.hcch.net/docs/d796ab1c-7137-4376-8564-3dcd7d078e49.pdf
The Convention is by definition in force between all the States that have ratified the Convention.
If a State has become a member through the accession procedure of art. 38 of the Convention, but has not ratified the Convention, you must check if the other State has accepted the accession of the other state. If one State has acceded before the other State, the first State must accept the accession of the second State.
If the home country of the child is not a member, but the country where the child is currently staying is a member, you can still use the Convention and ask for a return decision in the country where the child is staying.
The U.S. State Department keeps track of countries that enforce the Convention and maintains a similar website: https://travel.state.gov/content/childabduction/english/country/hague-partycountries.html
So you will have to check the date of accession, date of ratification and possible acceptance of the accession to find out if there is a treaty basis between the two countries.