Unfortunately, there is rarely a single sentence that will say "The inventive concept is X".
The abstract of a patent document is (in the U.S.) supposed to be a short description of the invention, but is not intended to have a lot of details about the specific invention. Abstracts are typically not very helpful.
In U.S. patents, the first section of the disclosure is usually the background. These can be helpful explaining things that are similar to the invention. Sometimes, the last paragraphs of this segment will have statements that try and explain what is wrong with the prior art and what the invention intends to correct/improve/fix. Again, there isn't a lot of detail about the exact invention, but it can helpful in finding the general concept for the patent.
Following the background is the summary of the invention. Some will have paragraphs that attempt to generalize the ideas and methods of the patent, but with varying levels of detail. Other summaries will be very generic restatements of claim language and won't be very helpful.
Later than that is the detailed description of the patent, i.e. the long, difficult to understand part.
At the end of the document (in the U.S.) are the claims. That's what is intended to be the invention and should include the inventive concept, but claim language can be challenging to understand.
Looking at the drawings can be useful. If you have knowledge of the technology related to the patent, you can pick out familiar elements in the drawings and attempt to guess what the 'inventive stuff' is based on what looks unfamiliar in the drawings. Also, matching things in the drawings to stuff that is in the claims can help with determining the invention.