@Wolff's answer is great and can be very useful.
Another way if you prefer to have the data determined without visually needing to verify anything is to use Preflight in Acrobat.
I'm using Acrobat Pro X here, merely because I prefer that version. Things should be very similar in newer versions of Acrobat.
With a PDF open, choose
View > Tools > Print Production > Preflight from the menu.
You can use any of the options in the green box below to simply verify compliance with PDFX standards, which requires no RGB data to be compliant.
Once you run the compliance test, you can see easily if there is any RGB data and on what page of the PDF it is located. Below shows a non-compliant PDF containing RGB data on the left and a compliant PDF on the right.
If you don't have Acrobat Pro and are creating PDFs destined for commercial printing, you need Acrobat Pro. Similar to needing InDesign to create professional-level layouts, print production often needs Acrobat Pro for some professional-level tasks. This is one such use case.
Note: If you are using Adobe InDesign you can preflight with InDesign. This can be helpful to determine issues before you ever get to the PDF. Be aware though, even if you preflight in InDesign, the PDF Job Options used when exporting to a PDF can alter things. It's really best to preflight production PDFs using Acrobat, as opposed to preflighting layout files using InDesign.
I have no affiliation with the link above. It is merely posted for additional information.