I am trying to create my first medium-sized poster in Photoshop, to print at 300ppi on A3. I need to bring together a collage of small files but I am worried that I will inadvertently expand one of the images beyond the 300ppi mark, so it looks blurry. In particular I am using iPhone and Android screenhots and I want the text to appear sharp. Can you advise - is there something like the pre-flight tool in InDesign for checking resolutions of individual components?
Since they're screenshots they'll be well below the 300ppi anyway so what you need is when you use them for them to be scaled using "nearest neighbour" mode (to give you sharp pixel edges.) My usual workflow is to scale the sources from their current ppi to multiples of it (say 6x).
The reasoning is that the pixel edges from the source remain sharp - that way it won't look unnatural or blurry, at least in a way that's unnatural for the subject matter.
Also going under 300ppi isn't really a major problem for screenshots since the viewing distance for an A3 poster won't normally be close enough for it to be a serious problem.
I don't think the solution exists.
An alternate method to track the scale: In Photoshop CC/CS6, if you drop an image into the PSD file, it remains as an object/reference. If you free transform that object, it remembers the scale. If you go back and re-transform it, it remembers the percentage of it's original size.
Not exactly an 'Effective PPI', but at least it will tell you if you are stretching the image past 100%.
On a side note, InDesign also displays the effective PPI of your images in the Links panel as you are working.