How do you know what radioactive decay equation represents a a fusion reaction?

This makes no sense, since radioactive decay and fusion reactions are opposite processes.

In radioactive decay, a large nucleus splits into two smaller pieces (it may be two similarly sized pieces, or one large piece and one small one, such as in alpha or beta decay).

In a fusion reaction, two small nuclei (usually very small ones) join together to become one larger one.

Therefore, no radioactive decay equation represents a fusion reaction.

Two or more smaller atoms combining to make one bigger atom.