How to Ask Your Partner to Get Tested
The best time to discuss getting tested for STI's is BEFORE engaging in sexual activity. It is also important to share your sexual history with a new partner, so they are aware that you have had sexual relations with other partners in the past. It is not necessary to discuss exact numbers of sexual partners, since being tested ensures you are STI-free. Although it is important to tell a partner if you have or have had an STI in the past. Once you have talked about your past sexual history, it is important to explain that in order to make sure you are both safe, it is a great idea to go get tested together.
Even if a person is not in a relationship, they still may have sexual partners. It is important for a person to ask any new partner, relationship or not, when the last time they were tested was. To ensure they do not feel offended, let them know the last time you were tested first. If you do not trust the person's answer, it may not be a good idea to engage in sexual activity with them in the first place. Remember to use condoms or other barrier methods (STI's may still be transmitted through oral sex for lesbian couples) to reduce your risk of contracting an STI.
Source: Sexinfo online