We have enough forensic evidence and knowledge of genetics to infer that evolution occured over the eons with a very high level of confidence. That makes it a very strong theory. What we cannot do is go back in a time warp and absolutely confirm it by observing it in progress. If a creationist were to assert that an almighty supernatural creator made it happen all at once and faked the physical and chemical properties to create an illusion of evolution, science would have no means of refuting it.

Do not confuse my sense of a theory with what many disbelievers mean when they say "only a theory". They are thinking in terms of a wild guess or hunch. A truly scientific theory is developed by examining all available evidence, and is revised if necessary when additional evidence is found and critically analyzed.