How do you differentiate ultralights and experimental/uncertified from certified?

These days its very difficult to tell a 3.axid ULM from an experimental or certified aircraft. In fact many UL pilots have a PPL and fly them like they would any other aircraft. Its all about the MTOW, well almost, there are some other factors to be taken into account.
In France experimental are F-Wxxx until they become.F-Pxxx which encompasses all annexe
ULM are F- Jxxx.

In Germany, it’s relatively easy to see which aircraft are Ultralights or not, just by looking at the registration. Aircraft with a registration beginning D-E are Single Engine Piston, D-M registered – eg D-MVAN – are ultra light, D-K are Motorgliders….

It’s not possible by registration to differentiate between experimental and normal. However, ULs have LN-Yxx, gliders have LN-Gxx, helicopters have LM-Oxx. But then you can differentiate gliders and helicopters by the looks also Experimentals will always have EXPERIMENTAL printed in large letters in the cockpit and on the fuselage by the cockpit.

This is a start. See also “Threads possibly related to this one” below.

A few types can exist as certified or annex 1. We had a thread on types which are annex 1 (formerly called annex 2) but which don’t need permits; these can be valuable. Basically it is like e.g. an RV (can do own maintenance, etc) but does not need permits to fly internationally.

A few types can exist as UL or annex 1, depending on the country of registration. @io390 has one of these.

I am sure people can post examples, but in general there are no easy rules.

There’s an interesting and comprehensive list of international aircraft registration prefixes and their sub-categories on Wikipedia. Some countries are very specific, e.g. Germany with 15 different types, or Switzerland allocating letters to manufacturers. Nothing to add for Romania though:

YR-AAA to YR-ZZZ
YR-1000 to YR-9999 (Gliders and ultralights). Previously CV until 1929.
YR-D0000 to YR-D9999 (UAVs, drones).

Edit
@Laurent_N Does the Romanian CAA have an online registration lookup? E.g. the UK G-INFO register has a category for certified/permit. That might tell you straight away.

“Basically it is like e.g. an RV (can do own maintenance, etc) but does not need permits to fly internationally.”

Are you sure an LAA Permit aircraft can fly internationally without permission? There were country deals preBrexit, and probably today
While in EASA, my Bolkow was on an EASA Permit, and presumably could fly anywhere is EASAland. Some individual examples of Certified aircraft are on Permit.
It’s more of a mess than you think.
PS I didn’t know the Permit was EASA not LAA until Brexit.

The UK Annex 1 permit was LAA, unless it was a CAA permit like e.g. a Spitfire or a Lancaster was.

EASA never AFAIK issued Annex 1 permits.

I finally found the thread about those rare types I was referring to.

“Experimental” is a term rarely used in Europe for the actual licensing category. It may exist in some countries for what is basically Annex 1 but most people who use it in Europe picked it up from the US scene.

Another approach to the OP’s question would be to search for the EASA Type Certificate. Then search for the FAA Type Certificate. There are a few types which have one or the other but not both. If a type has neither then it is simply not certified (in this context). Norway is, of course, different…

If you are a spotter, or someone sitting in the airport tower, in the vast majority of cases an Annex 1 or a UL is obvious from the type, and there aren’t all that many different types flying in significant numbers.

If not looking for accuracy, a 2-seater is likely to be uncertified unless it is one of the well known ones like a Cessna 150, and a 4-seater is likely to be certified. That is just how the market has developed.

However, a number will not be obvious at all.