How does one diagnose whether they have it orally?
Herpes outbreaks are not necessarily apparent visually, which is true for both hsv-1 AND hsv-2 ("genital herpes").
The only way to really know if you are infected is to get tested during an outbreak, which is not always easy to do as outbreaks do not always correspond to visual cues. So you either need to trust the person you're intimating with or you need to take precautions, which also don't always prevent infection. Condoms reduce the transmission rate, for example, but don't preclude transmission to areas of the body not covered by the condom.
Second, you're being downvoted because oral outbreaks can be very mild and may just look like acne, or a razor burn, or any number of other normal dermatological conditions. "Just looking for it" is perhaps the worst way to possibly think about any STD.
In response to another question in this thread, the safest way to "diagnose" herpes in others, is probably to ask them. If they don't know (haven't been tested lately), assume they're infected. And if you can't trust them to tell the truth, you probably have no business being (physically) intimate with them... (Not meant as a moral judgement, just as an observation).