Most of us know which ones will bottom feed, from research and use, so you can ask.
Also, which ones you can get a BF center pin for.
Another way is read the description carefully, many will say so these days.
"The Charm Style RDA is with top heat sink design and in 21mm diameter. The RDA supports top & bottom adjustable airflow control, 2 air slits on the sleeve. This RDA comes with botton feeder pin."
Some RDA's don't mention it, but have a center pin with a tiny grub, so you can use as a dripper.....remove the tiny grub....use for squonking. The Velocity V2 is one, but not every clone has one.....the Tsunami 24mm is another example.
All Goons will squonk if you buy the available center pin , and most Goon clones will use a squonk pin if they are 1:1
Good hex head squonk pins can be had on 3F Vape, and fit all of the Goons on their site EXCEPT the LP models, because they aren't 1:1 clones.
One of the greatest RDA's is the Velocity V2. You can use it either way, and they have the clones on FastTech, but you have to be careful which you buy. I can vouch for the one that sells for $9.57 and think it's item # 4491602 have over a dozen of them and can't tell the difference between an authentic.If you need the exact # just holler.