When you need to watch for something carefully, there are many different phrases you can use to express that idea. One common synonym for "look out for something" is "beware." This word has a bit of a warning tone to it and implies that there may be danger or risk involved. Another similar phrase is "be on the lookout", which is often used by law enforcement to ask the public to help them find a suspect. Other options include "keep an eye on", "pay attention to", and "take note of". The specific phrase you choose may depend on the context and the level of urgency you want to convey.