How to tell Simplified and Traditional Chinese apart?
Some regular simplification rules:

釒(金)→钅: 針→针, 鈴→铃
飠(食)→饣: 飲→饮, 飯→饭
糹(糸)→纟: 紅→红, 絲→丝
訁(言)→讠: 計→计, 語→语

貝→贝: 貨→货, 貴→贵
見→见: 視→视, 現→现
頁→页: 項→项, 題→题
東→东: 陳→陈, 棟→栋
車→车: 軟→软, 輩→辈
門→门: 問→问, 閑→闲
烏→乌: 鎢→钨, 塢→坞
魚→鱼: 漁→渔, 魯→鲁
鳥→鸟: 鴨→鸭, 鷹→鹰
馬→马: 媽→妈, 騰→腾

There are also some simplified examples of complex components:

爿→丬: 壯→壮, 將→将
豐→丰: 灃→沣, 艷→艳
風→风: 瘋→疯, 諷→讽
長→长: 張→张, 帳→帐
堯→尧: 燒→烧, 饒→饶
䜌→亦: 戀→恋, 彎→弯
睪→: 澤→泽, 驛→驿
炏→艹: 營→营, 榮→荣
蘭→兰: 欄→栏, 爛→烂
羅→罗: 蘿→萝, 邏→逻
買→买: 買→买, 賣→卖
產→产: 鏟→铲, 滻→浐
齊→齐: 擠→挤, 濟→济
幾→几: 機→机, 譏→讥
柬→拣(the right part): 煉→炼, 練→练
盧→户: 廬(广+～)→庐(广+户), 驢(馬+～)→驴(马+户)
盧→卢: 鱸(魚+～)→鲈(鱼+卢), 顱(～+頁)→颅(卢+页)
戔→戋: 錢→钱, 棧→栈
咼→呙: 鍋→锅, 窩→窝
樂→乐: 爍→烁, 櫟→栎
豈→岂: 鎧→铠, 凱→凯
肅→肃: 蕭→萧, 嘯→啸

There are also many irrational simplifications, such as cursive writing, or homophonic substitution, and some characters are combined by these simplified components. If you see Chinese characters with a lot of these parts, you can be sure that this is traditional Chinese.