This tool only works for rules in which there are only 2 cell states. For example, it would work for Life but not for Generations rules. It only searches for the pattern exactly as it is, not including rotations or reflections. It also doesn't search for different phases of an oscillator. To use this script, put the pattern you are searching for on the Layer above the Layer you want to search in, go to the Layer you want to search in, and run the script. It will give the output on the status bar.
Hopefully, this functionality will be included in Golly in the future. Note that I would be honored if the code here is used for this purpose. For now, I hope this tool helps.