Well now Maoraigh, there are quite a few permit aircraft* in your locality with Mode S transponders, mainly TRIG units I believe, so I’m a little puzzled by your question.
As to checks, I presume the LAA wants to have details of any installation but with, for example a TRIG, I understand from TRIG themselves that there are no formal checks required, ever, beyond calibrating it against your altimeter. Whether or not your altimeter is accurate is a separate question, though.
*KIS, RV7, Jodel, Wagabond, Zenair, Eurofox, Fly-Baby & possibly a Jabiru.
ChuckGlider wrote:
s to checks, I presume the LAA wants to have details of any installation
There’s a couple of forms: one detailing all the radios in the aircraft (com, transponder, nav, gps) and an FT (flight test) Avionics form. The inspector signs the first form, the pilot can do the FT Avionics form. All the avionics in the aircraft must be tested after a change and put on the FT avionics form, but it’s basically as simple as going flying, talking to someone on the radio, and asking a suitable ATC facility what the transponder is telling them.
My post was in response to the regulation below, and was copied from another thread. Means of compliance was mentioned. Peter and I think this is an airspace requirement.
I have been flying P to F with Mode S for several years.
“AMC1 to CS-ACNS.A.GEN.010 requires a check of altitude correspondence and correct Mode S address transmission every 24 months for Mode S installations and this is in line with previous national requirements for Mode C transponders. We had a UK mandatory requirement which later became an EASA AD (AD 2006-0265), later replaced with SIB 2011-15R2 (http://ad.easa.europa.eu/ad/2011-15R2) which now recommends the 24 month testing.”
This thread is worth updating a bit.
Has anything changed in the last few years as regards ATC equipment so they can see your aircraft reg, for example?
I guess it depends on area – in Croatia they can see reg and altitude set at G1000 (probably also from other integrated avionics) maybe something in addition.
ATC in Poland (EPWW) see mode-s callsigns.
In Germany we can see the Mode-S-ID in most areas and altitudes. We can also see selected level, IAS, magnetic heading, baro vertical rate if transmitted and needed. Selected and cleared level are automatically checked against each other and an alarm will be triggered if they don´t match.
About all European ATC can now see Mode-S, even where I fly the most, in Ukraine. If you are flying cross-country in controlled airspace in Europe, Mode-S is pretty a given these days. Also, based on the US move to ADS-B, you can find many GTX330 on ebay for 500-700USD.
Swedish ATC can certainly see the selected altitude if you have a mode S transponder. (I fly an aircraft with G1000+KAP140 where the altitude select in the FMS is entirely separate from the altitude select in the A/P, so ask me how I know. 
)
I haven’t asked but it stands to reason that they can see the flight ID as well.
Can ATS see the tuned in frequencies via mode S?
