答：从形式和意义两方面去判断.
一、形式
虚拟条件句中的谓语动词必定是过去时态.主句必定是would+ do / have done 形式.（would根据人称和意思,可改用should （第一人称）,和could (能）,might(可能））
具体：
if从句 主句
与现在情况相反的假设：did would +V.
与过去情况相反的假设：had done whould have done
与将来情况很可能相反的假设：did/ were to+V./ should +V.would +V.
注：be动词用律用were
二、从意义上去判断.
象If I were you,I would do sth.很明显是假设的,因为我怎么可能是你呢?
主要还是要从形式上去判断.
如有人说：如果我有10万元,我就买一辆新车.这是很难判断这是虚拟语气的句子,还是真实的条件句.但是：
If I have 10,000 yuan,I will buy a new car.这就是陈述语气.
而：If I had 10,000 yun,I would buy a new car.就明显是虚拟语气.
