Neither, assuming your examples are complete.
There was someone at the door. It was the milkman.
The past tense makes it hard to guess your context
If you use "is", it makes more sense if you are informing someone about the fact
"There is someone at the door. It could be the milkman" (if you are not sure).
"There is someone at the door. It should be the milkman" (if you expect him).
depending on what you meant to say.
Your sentences could be fixed if you add a condition
There was someone at the door. It WOULD BE the milkman by the sound of the clinking bottles.
There was someone at the door. It WOULD HAVE BEEN the milkman, had he not broken his leg the day before.
If the person is alerting the fact that there is someone at the door now and they are expecting the milkman, then the sentence is as follows, where the "would" is used to show you expectations
"There is someone at the door; that would be the milkman."