OPINIONDoes it use words like “best,” “better,” “most important,” or “should”? Those answers depend on values, not experimental data.
EXPLANATIONDoes it start with “How does” or “Why does/do”? Those ask for a mechanism — not the outcome of an experiment you could run.
PREDICTIONDoes it ask whether something “will ever” or “will scientists” do something in the future? You cannot run an experiment about what will happen decades from now.
TESTABLEA testable question names what is being changed and what is being measured — and you could actually run the experiment today.