Error: "The certificate cannot be dropped because one or more entities are either signed or encrypted using it."
To find items associated with Certificates and Asymmetric Keys, first try the query posted in this DBA.SE Answer:

Find signed procedures, functions, triggers, assemblies and by which certificates / asymmetric keys

If that does not return any objects, next try the following queries which look for:


Logins
Users
Service Broker Endpoints
Database Mirroring Endpoints
Symmetric Keys
Database Encryption Keys (used for TDE)


Please note that Logins are Server/Instance-level while everything else is Database-level. Also, Database Encryption Keys, while being at the Database-level, are reported in a DMV that returns data for all databases, and so does not change based on the "current" Database.

-- Server / Instance Logins (results not sensitive to local / current Database)
;WITH certs_n_keys AS
(
  SELECT 'Certifcate' AS [Type], crts.name, crts.certificate_id AS [cert_or_asymkey_id],
         crts.principal_id, crts.pvt_key_encryption_type_desc, crts.[sid],
         crts.thumbprint
  FROM   [master].sys.certificates crts
  UNION ALL
  SELECT 'Asymmetric Key' AS [Type], asym.name, asym.asymmetric_key_id AS
         [cert_or_asymkey_id], asym.principal_id, asym.pvt_key_encryption_type_desc,
         asym.[sid], asym.thumbprint
  FROM   [master].sys.asymmetric_keys asym
)
SELECT cnk.*, '---' AS [---],
       sp.[name] AS [PrincipalName], sp.principal_id, sp.type_desc,
       sp.create_date, sp.modify_date
FROM   certs_n_keys cnk
INNER JOIN sys.server_principals sp
        ON sp.[sid] = cnk.[sid];


-- Database Users
;WITH certs_n_keys AS
(
  SELECT 'Certifcate' AS [Type], crts.name, crts.certificate_id AS [cert_or_asymkey_id],
         crts.principal_id, crts.pvt_key_encryption_type_desc, crts.[sid],
         crts.thumbprint
  FROM   sys.certificates crts
  UNION ALL
  SELECT 'Asymmetric Key' AS [Type], asym.name, asym.asymmetric_key_id AS
         [cert_or_asymkey_id], asym.principal_id, asym.pvt_key_encryption_type_desc,
         asym.[sid], asym.thumbprint
  FROM   sys.asymmetric_keys asym
)
SELECT cnk.*, '---' AS [---],
       dp.[name] AS [PrincipalName], dp.principal_id, dp.type_desc,
       dp.create_date, dp.modify_date
FROM   certs_n_keys cnk
INNER JOIN sys.database_principals dp
        ON dp.[sid] = cnk.[sid];


-- Service Broker Endpoints
SELECT crts.name, crts.certificate_id, crts.principal_id,
       crts.pvt_key_encryption_type_desc, crts.[sid], crts.thumbprint, '---' AS [---],
       endpts.*
FROM   sys.certificates crts
INNER JOIN sys.service_broker_endpoints endpts
        ON endpts.certificate_id = crts.certificate_id;


-- Database Mirroring Endpoints
SELECT crts.name, crts.certificate_id, crts.principal_id,
       crts.pvt_key_encryption_type_desc, crts.[sid], crts.thumbprint, '---' AS [---],
       endpts.*
FROM   sys.certificates crts
INNER JOIN sys.database_mirroring_endpoints endpts
        ON endpts.certificate_id = crts.certificate_id;


-- Symmetric Keys (scroll results to the right to see Key name)
SELECT crts.name, crts.certificate_id, crts.principal_id,
       crts.pvt_key_encryption_type_desc, crts.[sid], crts.thumbprint, '---' AS [---],
       ncrptns.*, '---' AS [---], symkys.*
FROM   sys.certificates crts
INNER JOIN sys.key_encryptions ncrptns
        ON ncrptns.[thumbprint] = crts.[thumbprint]
INNER JOIN sys.symmetric_keys symkys
        ON symkys.[symmetric_key_id] = ncrptns.[key_id];


-- Database Encryption Keys (for TDE; results not sensitive to local / current Database)
SELECT crts.name, crts.certificate_id, crts.principal_id,
       crts.pvt_key_encryption_type_desc, crts.[sid], crts.thumbprint, '---' AS [---],
       DB_NAME(dbkeys.[database_id]) AS [DatabaseName], dbkeys.*
FROM   [master].sys.certificates crts
INNER JOIN sys.dm_database_encryption_keys dbkeys
        ON dbkeys.[encryptor_thumbprint] = crts.[thumbprint];