How to check the recurrence intervall of heavy rain events
In engineering it is common practice to build intensity-duration-frequency (IDF) curves, which basically map out the frequency of a given rainfall event based on its average intensity (mm/hr) and duration of the event. This addresses the issue of needing to classify a 'heavy' rainfall event. In general, the duration and intensity are inversely related (more intense rainfall events tend to last a shorter time).

There are many examples of calculation and software packages to actually build these curves, which are used often for engineering design to help decide the storm a given piece of infrastructure is being designed to (e.g. 5 year storm, 25 year storm, etc.). Most municipalities will have developed their own IDF curves to set standards for design. 

There are also examples of online tools to help generate IDF curves for a given location, such as this one in Ontario. This tool is neat as it calculates the IDFs for a given year, and uses trend analysis to predict future changes to the IDF curve, allowing you to also pick the target year for generating the IDF curve.

If you see that the intensity/duration changes for a given frequency storm event (e.g. 50 year), that would indicate a shift in the rainfall patterns due to climate change. You may need additional statistical tests to check if these changes are statistically significant (examples of this type of analysis applied here and here). You will likely not see much change in these curves over a few years as the datasets overlap quite a bit and the trends will not change rainfall patterns in that short a time frame, however, over a 25 or 50 year period you will likely see some changes in the IDF curves, depending on the location.