How do you specify past tense for 是?
You have to remember, unlike in English, we do not have past tense for verbs in Chinese grammar. Both "is" and "was" is written as "是" in Chinese.

"是" in "她是好老师" could be "is" or "was". We really don't care which, because it is presumed readers can find out the sentence is in past or present tense by looking into the context.

For example: 


"她以前是个好老师"(she was a good teacher before)
"她曾经是个好老师" (she was a good teacher once)
"她三年前是个好老师"(she was a good teacher three years ago) 


All three example sentences above clearly indicated the verb "是" is in past tense. 

If no time reference is found in the context, then all verbs are in present tense by default.                 

When you are reading a historical novel, even most individual sentences in it didn't have time reference, you should still know it is in past tense; If you are reading an instructional manual, you know the text is written in present tense entirely.                


  The answer to "How do you specify past tense for 是 " should be "there's no need"