Lenka,
visual inspection is typically not the method of choice for verifying presence or absence of residuals, but typically chemical material characterisation studies according to ISO 10993-18. You could avoid/complement these by gathering comprehensive foil material data from your supplier, i.e. base material(s), absence/presence of any additives (such as denesting additives, colourants, coatings, ...) and its manufactrung process. In case you succeed in gathering these data comprehensively (i.e without significant "unknowns"), you could also be in a position to assess whether you could exclude any unacceptable risk from residuals, without having to generate additional test data on your won.
BTW, a passed , standalone cytotoxicity test would only tell you that the test material has no cytotoxic potential, not more.
HTH,