not really understanding what you're trying to do. Maybe rephrase what you said above.
If you don't want them to make changes to the parts save off as a new file name, or don't take back their "changed" parts.
Like @dan, I'm not clear of your question
Are other people the users of the PDM you're using? if not, why you worry if the components are not checked out
If you are sending parts to people by email, or any other way outside of the vault, then you are inviting problems.
If you have ownership of the part in PDM, nobody else should be able to check in any changed parts.
If you are trying to check it back in, after getting it back from someone you shared with outide of pdm, then you aren't using pdm correctly.
One issue is that if they open it on a different version of Solidworks, then pdm will think the file changed, even if they din't do anything, besides open it and look at it.
If they printed it to pdf, or their printer, and then saved it, solidworks pdm will recognize it as being changed.
PDM rules exist for a reason, and sharing files outside of the pdm system just bypasses them.
If you have a file Checked-Out in PDM you can perform a Drag-&-Drop (or Cut/Paste) from your external folder into the PDM folder where the file already exist. The files need to have the exact same file name, then Windows will overwrite the checked-out file in PDM (you'll get a warning). Once that is complete you can check-in the file. This will preserve the history in PDM for that file.