Bigger concern than fabric is the wing spar, especially if you plan to fly acro. If the wings are original then the spar is wood. That is not necessarily a bad thing, but you do need to understand the risks you are buying into.
The FAA published an AD about 20 years ago that affects all 7 and 8 series aircraft with wood spars. It requires a detailed annual inspection to look for cracks in the spar, and the installation of a considerable number of access holes to conduct the inspection, especially above and below the front strut attach fitting.
The backstory on the AD was the FAA found several spars with considerable damage, mostly compression cracks, and there was at least one fatal Citabria accident. The damage was caused by contact of the wing with the ground or stationary objects such as hangar doors, not by aerobatics. It is important to state that the Decathlon wood spar is substantially more robust than the Citabria spar, and there is no documented case in the NTSB files of a Decathlon wood spar failing in flight, other than due to greatly overstressing the airframe.
There are different views on wood spars in the Decathlon community. Some buyers will only consider metal spar aircraft, which would be any produced after 1992, plus any prior aircraft that have been retrofitted. Factory metal wings cost about $35K, so that is an option. Others are comfortable with wood spars, as long as they have been inspected. I count myself in the latter group. I compete in IAC Sportsman acro and pull 4-4.5G in practice sessions several times a week, and have zero concerns with my wood spar.
So I would look for a few things in your prospective aircraft: A) have the access holes to do the spar inspection been installed; B) does the aircraft have a track record of regular AD compliance over the last 20 years; and C) is there any physical or logbook evidence of a ground loop accident? If no red flags there, get a prebuy by someone who is familiar with the wood spar AD compliance, and pay the extra $$ to have them look carefully at the spar, especially at the strut attach points and doublers. If everything is good and the price is appropriate for a wood spar aircraft, buy it and fly it with confidence.
As far as the fabric cover on the fuselage and tailfeathers, conventional wisdom is that the fabric on aerobatic aircraft should be removed every 20 years or so to inspect the structure underneath. I think that is excessive. Again, I'm flying a 44 year old aircraft with original cover, and it is just fine. You can adequately inspect the frame and internal components by removing the rear seat and sticking your head back in the battery compartment with a bright light. If you really want to get a good look, it is easy to pull the floorboards out, and even lay down and stick your head and shoulders into the tailcone. Also feel carefully along the bottom longerons on the fuselage tailcone for bumps, which are signs of rusting. The wood formers and stringers are probably warped, but IMO those are mainly to establish the shape during covering.
If your cover is original, the covering system is Ceconite and Dope. You can verify it is dope by wiping a Q tip with some lacquer thinner or acetone on the finish. If the finish comes off readily, it is dope, which is just lacquer paint. That is good, because dope is really easy to repair. I would find someone who knows dope and fabric to take a look and assess how much life you have in the fabric. If you have tapes popping, or large areas where the coating has cracked and exposed the fabric to UV, then you are going to need to do some spot repairs at a minimum. Again, dope is easy to repair. Scrape away the areas of bad coating, glue down any lifting tapes or patches, and spot paint with dope. Do NOT mix systems or use other paint products. Stick with the original Ceconite process.
If the bad areas are large enough, or the fabric has deteriorated due to exposure, at a certain point it is not worth the effort to repair it, and you are looking at a recover. I have seen quotes to recover the fuselage and tailfeathers ranging from $10K for IRAN to $20K for a full restoration (strip, bead blast and reprime the frame, and assemble with new wood, cables, etc). I think costs to recover the wings are similar. Materials cost is about half of that, so if you are handy and want to do it yourself, that is a viable option.
Some people will talk about a punch test. If I was selling my aircraft an someone wanted to poke a hole in it, they would get a punch test in their nose. Punch tests are for old school cotton. For Dacron (Ceconite), as long as the coating is intact, the fabric is fine. If the coating is cracked to the point where UV can get through, the fabric can deteriorate in that area. But there is nothing to prevent you from repairing that spot with a patch, as long as it is not too large.
