If you do it yourself wear gloves and you will notice that the bolts x 10 are not as tight as you would expect
16 bolts on ours, use a 10mm ratchet spanner, will take around 2 hours DIY if you take your time. 3+ hours with an ordinary spanner and, in my case, 3 months physio. Safety first, as mentioned, cover sharp edges and/or use gloves.
I found that the tell-tale sign of a sunroof leaking is the wet internal roof fabric at the top of the plastic trim at the front of the doors, and a small amount of water creeping over the plastic trim at the bottom of the door, over on to the carpet.
If you do not have a sunroof, you're looking for signs of leaking grommets in the bulkhead, leaking through the boot, around the door seals. Best way to find out where, is wait for a dry day and soak it with the garden hose, wipe down the outside then open up to find the wettest spot and work from there. You may end up removing a lot of the interior to find the leak. With AC, don't use it for a couple of weeks and see if it clears up, you're also likely to find it smells different but, without comparison, that may be difficult, my best description is mildew.
Enjoy, it's French, it's fun and can be very cheap but a pain in the butt if you don't keep on top of it. My last car, a 96 Passat was similar but the little things wouldn't affect it as much as they do in our Clios.