I've sat here trying to think this one through and managed to over-think it.

Please can someone advise, is it okay to say that 'she laid a tentative hand on his shoulder' or do I have to show the tentative and/or resort to saying 'she tentatively laid her hand on his shoulder'?

I want the first, if possible.

The first is absolutely fine, honest.

I agree, I think the first is better - it brings a much clearer image to my mind.

YEs, I really like it - very economical way of evoking both the physical quality of the action, and the emotion underlying it.

Which is why separating things into either Showing or Telling is so often so unhelpful.

Thank you all.

Strange how when you stop and think, everything can become so much murkier!

I agree, definitely the first one. The image combined with the emotion says it all succinctly.