▲首先注意：in future不一定指“从现在往后”。它可以用于过去将来时，意为“从那以后”，“从当时那个时间往后”。例如：

I asked her to be more careful in future. 我要她以后要更加小心。

He would never make such a mistake in future. 他从那以后决不再犯如此错误。

包括你的例句：

In future, I would make sure I'd checked out whoever was fitting it with their previous customers.

▲在would make sure的宾语从句中，had checked out 属于“过去的过去”，说明发生在would make sure 之前。而在checked out的宾语从句中，使用过去进行时 was fitting，不用现在进行时，因为从句的时态受主句制约。我们不能说：He asked me what I am doing.  应改为was doing。