The Script
https://www.zevrix.com/linkoptimizer.php
Not sure if this one converts colours? https://www.kasyan.ho.com.ua/resize.html
InDesign
Well in CS4 I think it gives little icons in the Links panel to indicate what colour space an image is. Which is usually the first clue 
Before CS4 the only way to tell was to go to an image, open the Links Panel and go to Link Information from the fly out menu – the colour space is usually told there for raster images (not so much for vector).
Photoshop
If I wanted to make all my images CMYK I'd setup a Batch Action in Photoshop to convert all the images in my Links folder to convert all Images to CMYK – check out these instructions on how to setup a batch you can use whatever commands you want in this case you want Image Mode>CMYK for your action – then run the batch action from the File menu and select your folder you want to convert (make a backup of your images first though just in case something goes wrong) https://www.ehow.com/how_505052…..op-cs.html
Adobe Bridge
You could open your links folder through Adobe Bridge and add the Colour Mode into the list information to view through the Preferences –

Convert Through InDesign
Ok so when you output the PDF you should be able to go the Output options and then select a destination profile
The destination profile should convert all your images to CMYK. You just need to select the right one for the right job.
There are Euroscale Coated and Uncoated (Coated and Uncoated refer to the stock of paper – you can check with your printers)
Then there are the Coated Fogra and Uncoated Fogra – again check with your printer.
And there is US Web Coated and Uncoated (for a web printer like they print newspapers with)
US Sheetfed Coated and Uncoated
These are things you should check with your printer – but I usually go with Coated Fogra 39 or Uncoated Fogra 27

Can't really think of other ways I'd go about it?
Generally I don't worry too much about it – unless it was an Art book or something with really fine quality that required a lot of control – I'd usually use RGB images in InDesign and convert to CMYK on output.