Learn Finnish: Minä epäilen, että mies on lääkäri. - I suspect that the man is a doctor.

It actually means both! This can be tricky for English speakers. You have to rely on context to tell them apart. If you want to make it clearer that you doubt something, the second part of the sentence (the että clause) is often negative. Since this sentence is a straightforward positive statement, "suspect" is the most natural translation here.

Do I always need a comma before että?

Yes. In Finnish, you must use a comma to separate the main clause (Minä epäilen) from a subordinate clause starting with että. While in English we often skip the comma and even drop the word "that" entirely ("I suspect the man is a doctor"), Finnish strictly requires both the comma and että.

Why are there no words for "the" or "a" before mies and lääkäri?

Finnish does not have articles like "a", "an", or "the". The phrase mies on lääkäri literally translates to "man is doctor", but based on context, it is easily understood in English as "the man is a doctor".

Why is lääkäri in its basic dictionary form here instead of the partitive case?

When you use the verb olla (to be) to state what a person is or what their profession is, both the subject (mies) and the description (lääkäri) stay in the basic nominative form. You do not use the partitive case when identifying a single person's profession.