There are certainly a lot of bad "flashes" on 4chinz that are just re-converted videos, yes, but a lot of them have cultural significance and are well known and appreciated like MOU IKKAI.swf, 2hu stuff and even gachi vids.
On a side note, modern gachi and 2hu vids also need to be screened through before posting, since they are getting more popular with normalfags. Ricardo Milos is the one who got relatively recently popular even though he always was just a background character to Aniki and Van. And with touhou it's just normalfag references being forced in like degenerate mumble rap along with sans undertale or some fucken shit.
Back to the topic at hand, swf related is an absolute classic, even though it's just a video embedded in a flash container. >how to tell which videos are good and which are bad?
The only advice I can give on this possible question would be
L U R K M O A R
it's just something that you gotta feel for, anon.