Unfortunately the only way I know how to do it is measure some known length off of the drawing once you bring it in to InDesign, do a little algebra and scale it that way. The PS image doesn't have a scale unless you were super careful about that in the scanning. I.e. when you change the resolution at scanning etc it changes the native size of the image. I've never noticed any rhyme or reason to the size of the image when it comes in to InDesign, although I'm sure there is one. I think this is a case of a little elbow grease. Still easier than it was even 10 years ago.

if the scanned images are the correct scale, placing them in InDesign should not change the scale, I guess. Maybe save them in PS as PDF before bringing them in InDesign. Also check if the dpi of the image is the same as your indesign document, might help. If every image has the same wrong scale, you can once calculate in PS the correct scaling and record this operation and automate it, with just one click. good luck

Do all your scaling in Photoshop and place the PSD files in InDesign; it's best to not adjust image sizes in InDesign. If you set the correct size in Photoshop it should come in correctly in InDesign (i.e. 6x8 @300dpi); it reads the size and resolution info from the image (the dpi settings in InDesign are for export/printing)

Thanks for your help, I wouldn't have found that out on my own! Thankyou

you may be getting various results placing in indesign if you click and drag rather than just a single click to place. Single click to place should retain whatever image size you had specified in PS (or whatever), while clicking then dragging will resize your image based on the window you've just created.