On my 2 trailers, there is a decal with the number on it. If you have an old trailer the decal may be missing.
In Michigan the vin number is on the title and registration.
FYI - If anyone has an older trailer and the decal is starting to fade, you can order a new one (Decal) from the manufacture. I rebuilt/repainted my Eagle boat trailer last year and I call up Eagle and they sent me all new decals for everything + the paint code formula.