SE support is mandatory. DE support is optional. Therefore, when an implementation supports DE it must also support SE.
Some implementations might only support one model at a time for a given AS/ASP, but that is a (minor) limitation of that implementation.
There is no need for negotiation: all implementations supporting IPSP MUST support SE: so just use that.
Nevertheless, when an IPSP sends an ASP-UP message and receives an ASP-UP message in response, or sends an ASP-ACTIVE message and receives an ASP-ACTIVE message in response, it can determine that the peer is using DE mode and proceed accordingly.
It can also determine DE mode from REGREQ messages in a similar manner when registration is used.