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You have to think about an assumption as a missing premise that either supports or helps the conclusion, depending on if it's a good assumption (the inclusion of the assumption as a premise strengthens the conclusion) or a bad assumption (the inclusion of the assumption weakens the conclusion). Think of assumptions as premises with invisibility cloaks.

Basically, think to yourself, is this argument missing any logical steps to get from premise to conclusion?

Something to help you identify possible assumptions: You also have to remember that LSAT questions exist in a void; things that seem obvious to us because they are facts of the world WE live in can still be assumptions (i.e., the assumption that Tigers are mammals in the Tiger argument). You need to leave no stone unturned; don't take for granted the obvious assumption that Tigers are mammals. Train your brain to identify that assumption instead of permissively accepting that obvious assumption.