Where did you watch it?
Did you see it on a mainstream TV channel like Cartoon Network's Adult Swim block, somewhere online (i.e. a streaming service or YouTube snippet), or on a DVD/VHS?
What was the year of production?
If you're not certain, at least hazard a guess. Modern anime tends to use computer-based shading and dynamic lighting effects (and CGI) whereas older anime tends to be a flatter hand-drawn style.
What's the genre?
Obviously it's sci-fi or fantasy (or else you wouldn't be asking it here) but what else was it? Does it have elements of romance, is it a slice-of-life, family, coming-of-age, ecchi, etc?
What language did you see it in?
Was it dubbed or subtitled? Was the dubbing of a high quality (matching the mouth sounds) or low quality (basically just dubbed over)
If you've got a screen-shot of the show or one of the characters, where did you get that shot from?
Is the show set in the past, present or future? Was it set in Japan or another country (or outer space)?
Was there anything noticeable or unique about the animation style? Have you seen any other shows that look similar? Can you guess at the animation studio even if you don't know the show?
Where did you read it?
If you read it in paper format, where did you get it from? If you read it online, which site did you see it on or what sites do you regularly use?
Was it a drawn comic or a Japanese Manga? If you're not sure, it might be helpful to know that western comics tend to be in colour whereas manga will tend to have colour pages but the majority in black and white. Manga will tend to be released in short books (called Tankōbon) whereas western comics will tend to be on flimsy paper. Japanese manga will, even if translated, generally retain their distinctive right-to-left reading order rather than being mirrored.
Manga & Anime?
Are you aware of the manga also being made into an anime?
General helpful stuff
What were their ages? What were their names? Did they have any distinctive features, notably eye-colour, hair colour and/or defining marks on their face, body or clothing?
If the anime or manga features fantastical animals or monsters, what did they look like? What were they called in-universe? Were they summoned? Did they have any unique powers of their own? Are they intelligent and sentient?
Who do you think the property is targeted at; Kodomomuke (colourful shows and manga aimed at children aged 3-12) Shōnen (shows for younger males aged up to 15), Seinen (shows aimed at males aged 15-30), Shōjo (shows aimed at girls aged 10-18), Josei (dramatic works aimed at adult women aged 18 to middle-age), Ecchi (mild, often comedic adult content typically aimed at males aged 16+), or something else?