I was arrested for DV but case was rejected by DA, does it automatically state on my background check - detention only?

Does your criminal history automatically state "detention only?" No.
To get a certificate stating this, you would need to ask the police under Penal Code 851.6(b).
You can order a copy of your rap sheet at http://oag.ca.gov/fingerprints/security.

You should be eligible for an expungement. Contact a local attorney.

You're not eligible for an expungement in the state of California-- there's nothing to expunge. What you should do is consult with an attorney (most of us on Avvo offer free consultations) about potentially filing a motion to have the arrest record sealed and destroyed pursuant to Penal Code section 851.8. Those can be tough to get granted but it might be worth a shot.

I agree with Mr. Hill that you need to get a certificate of detention from the police in order to get a court order to seal the records. The problem is that the police may not agree to issue the certificate without proof that your case was rejected and, if the case was rejected, there may be no court order you can offer as proof. You may need to get certification from the DA that your case was rejected. Eventually, you will need to go to court anyway to get the order sealing the arrest record. You should hire an attorney to assist you with the court proceedings and help push through the bureaucratic red tape.

Once the statute of limitations for the charge of arrest has passed -- a critical and non-negotiable prerequisite -- your CA Dept of Justice record as reported via Live Scan report will indicate "detention only."
You do not need to do anything to cause your arrest to be a "detention only." That happens as a matter of law:
CA Penal Code § 849.5: "In any case in which a person is arrested and released and no accusatory pleading is filed charging him with an offense, any record of arrest of the person shall include a record of release. Thereafter, the arrest shall not be deemed an arrest, but a detention only."
If you wish a record of the fact of the detention only, follow the procedures of CA Penal Code § 851.6. The certificate specified therein is not optional with the arresting agency, but sometimes you have to work your way up the chain of command -- by letter or in person. I instruct my State licensing clients to obtain these certificates as a matter of course and I have never had a single failure or refusal where the statutory conditions are met.
CA Penal Code §851.6. "(a) In any case in which a person is arrested and released pursuant to paragraph (1) or (3) of subdivision (b) of Section 849, the person shall be issued a certificate, signed by the releasing
officer or his superior officer, describing the action as a detention.
(b) In any case in which a person is arrested and released and no
accusatory pleading is filed charging him with an offense, the person
shall be issued a certificate by the law enforcement agency which
arrested him describing the action as a detention.
(c) The Attorney General shall prescribe the form and content of
such certificate.
(d) Any reference to the action as an arrest shall be deleted from the arrest records of the arresting agency and of the Bureau of Criminal Identification and Investigation of the Department of Justice. Thereafter, any such record of the action shall refer to it as a detention."
Most arresting agencies will not issue the certificate on the basis of a written "DA rejection." Why? because the statute doesn't require that.