How do you spot a foreigner?
I have compiled a list based on my observations over the years. Of course, there are exceptions to these rules!

1. If a Japanese character has blonde hair, the gaijin's blonde hair will be a different shade
2. Even though English is his native language (usually) he will be unable to pronounce things properly and burst into random phrases such as "Oh my god!" and "Wow!"
3. Though the show may have aliens, robots, monsters from another dimension, or some other odd creature, the gaijin will be stranger than them
4. Roses are a frequent accessory
5. Half-Japanese will usually have reddish or blonde hair and act in a noticeably different manner than "normal" Japanese (ex: wild temper). Half-French and half-American are popular
6. French characters come from ridiculously rich families
7. Loudness and aggression are second-nature, much to the annoyance of others
8. The presence of a gaijin calls for immediately freaking out. Japanese females tend to swoon for a male gaijin
9. Despite efforts, they will not win over a potential romantic interest - in the end, it will go to a Japanese character
10. They will almost never have a significant role in the series (ex: help defeat a bad guy), and seem to only be there to stir up drama

For more information check out the example section.