mjhjmtu

I suspect that the used bike I want to buy might have a tubeless tires, but one is flat. Is there any external feature to look at that would allow easily to tell this for sure?

I have no experience with tubeless tires and would not know how to replace it.

While your question of identifying a tubeless tyre has been answered, I think that might not actually be what you have to worry about. A bike might come with tubeless tyres, but that doesn't mean that they're set up tubeless. The tyres might still have a tube in them, so your concern should be identifying whether there's a tube inside or not. The easiest way to do that is to look at the valve stem, because a tubeless valve will almost always be secured with a substantial lockring, whereas a tube with a presta valve might have a little silver lockring or usually nothing at all. Any wheel with a Schrader valve is almost certain to have a tube in it as tubeless Schrader setups are extremely rare. This isn't foolproof, because some people will put a big lockring on a tube and some might put a small one on a tubeless valve, but I'm confident that this will be accurate over 95% of the time.

Schrader valve:

Presta valve (tube):

Tubeless presta valve:

Look at the tire sidewall for model/name, tubeless tires usually have TL, UST, TR or Tubeless in it; some tires don't (like Schwalbe Pro One), so google the model and see if it's tubeless or not. Keep in mind that some MTB tires can be setup tubeless regardless of what manufacturer claims, in this case you can unseat some tire bead and check if there's tube or sealant residue inside.

For a tubeless setup you need three things: the right rim, the right tire, the right valve-stem. Carbon side up shows how to identify the valve stem, and the tire and rim should be marked, but you need to check all three, because having tires and rims rated for tubeless may be used with a tube (I do this all the time on riding lawn mowers because their rims suck and leak even though designed for tubeless operation.)

However in your case there is an easy check since there is a flat tire. First look at the valve stem. If it is a presta use your fingers to loosen the stem nut (skip if it is too tight to loosen by hand), then wiggle the tire relative to the rim while watching the valve stem. if wiggling the tire moves the valve stem you have a tube if you feel the tube stretching against the valve stem, you have a tube, otherwise either you have a tubeless setup or a burst tube.