I couldn’t find an obvious way of showing a trill mark, after I had hidden it with the “Show Trill Mark” switch in the Properties panel.
I ended up having to Filter the Notes, and Cut and Paste them back.
No, at the moment there’s no signpost for a trill if you end up hiding both its wiggly line and the ‘tr’ mark. Try not to do that 
Ha! I’ve been using hidden trills, combined with an “Editorial Trill” in square brackets, made from a Performance Technique. It works rather well, apart from this drawback.
A possible workaround can be to temporarily change the trill interval appearance in engrave mode / ornaments to auxiliary note then select the auxiliary note of the hidden trill and unset the show trill mark property. This of course only works if the hidden trill didn’t have a property override for interval appearance.