How to use "whether" in Chinese - do you need to say 如果？
No, you can't use 如果 here. You got the right syntax in there though. What you want is just 我问他有沒有见过马克 - "I asked him whether he has met Mark". Or, since you used 认识 for your translation in the question, 我问他认不认识马克 - "I asked him whether he knows Mark"

There's no single "whether" in Chinese; you have to create the appropriate term based on the situation. One way to do this is using the structure "(verb) 不 (same verb)". For example, repeating 有 to form 有沒有, meaning "whether (subject) has". When the verb is composed of two characters, such as 認識, normally only the first character is used before 不, hence 認不認識 for "whether (subject) knows".

Alternatively, another way of saying "whether is to use "是否 (verb)". For example, using this format, "I asked him whether he has met Mark" becomes 我问他是否见过马克. Similarly, "whether (subject) has" becomes 是否有, and "whether (subject) knows" is 是否認識. Saying "whether" in this style is somewhat more restricted to written forms of communication, but remains quite current.