As I mentioned a couple of weeks ago, Chinese characters are not unstructured scribbles, but formed from specific strokes made in a specific order. I hinted at another aspect of the structure of these characters a couple of weeks before that, too, when I discussed 魚 (yú), the character for fish.
As a reminder, here are four of the words mentioned in my 魚 post:
|魷魚||yóu yú||squid or cuttlefish|
|鯽魚||jì yú||tilapia or crucian carp|
|鱔魚||shàn yú||swamp eel|
If you look at the left-hand sides of 魷, 鯽, 鰻, and 鱔, you'll see what I described as "a sort of squashed version of 魚" — this is in fact what's known as a radical. Basically, a radical is the means by which a Chinese character is indexed (and thus located) in a dictionary. The radical often appears on the left-hand side of the character, but it may also appear in other positions. Note that it's an intrinsic, inseparable part of the character, not a prefix or suffix that can be left off.
When you're trying to identify a character you've seen on a menu, you can narrow your search down considerably if you can recognise its radical. My favourite way of searching by radical is the CantoDict radical search, but you may prefer the mandarintools.com version.
Generally, to look a character up by its radical, you'll need to know the number of the radical. There are 214 radicals in all, some used more commonly than others. The Wikipedia list of radicals points out that seven of them are used in more than 1,000 characters each, so these are well worth getting to know. As well as the seven mentioned there — 艸/cǎo/grass, 水/shuǐ/water, 木/mù/tree, 手/shǒu/hand, 口/kǒu/mouth, 心/xīn/heart, and 虫/chóng/insect — I also find the following crop up quite often in the characters used on menus:
|Radical 86||火 (hǔo/fire)||e.g. in 炒 (chǎo/to stir-fry), 炸 (zhà/to deep-fry), 煮 (zhǔ/to cook or stew), 熱 (rè/hot)|
|Radical 130||肉 (ròu/meat)||e.g. in 肺 (fèi/lung), 肚 (dù/dǔ/tripe), 腐 (fǔ/beancurd)|
|Radical 164||酉 (yǒu/wine)||e.g. in 酸 (suān/pickled), 酥 (sū/crispy), 醬 (jiàng/jam or paste)|
|Radical 195||魚 (yú/fish)||see examples above|
You don't need to worry too much at this stage about memorising radicals. Just be aware of their existence, and eventually you'll start noticing patterns in the characters that you see showing up often.
It's important to remember that while the radical can give you a clue as to the meaning of the character, it's not guaranteed to have anything to do with the meaning. The only thing you can rely on a radical to be is a way of organising characters in a dictionary, similar to alphabetical ordering.