X.509 certificate pkcs#1 v2.1 support

when using X.509 and the algorithmIdentifier of the publicKeyInfo is
rsaEncryption (it must has null params) how can one know is to use
pkcs#1 v2.1 or pkcs#1 v1.5?

accroding to RFC 4055, section 4:" Entities that use a certificate with
a
publicKeyAlgorithm value of rsaEncryption SHOULD use the default set
of parameters for RSAES-OAEP-params. "

unless the two algorithms pkcs#1 v1.5 and RSAES-OAEP with default
params are equal.

Thanks in advanced for your reply.