Does anyone know if there is a for sure (with the exception of some irregulars, of course) way of telling whether a verb is weak or strong?
Like BadDoggie said, you ultimately have to memorize for each verb whether it is weak or strong. However, if the related English verb is irregular (go/gehen, bring/bringen), the German verb is usually also strong.
Also, at least in English, the strong verbs tend to be ones that are used often.
But in the end, knowing that a verb is strong is not enough - you've got to memorize the conjugation.
Additionally, when conjugating verbs with a separable prefix into the present perfect, how can the correct ending, be it 't' or 'en', be determined?
I think that the conjugated endings of separable verbs are always the same as those for the base verb.