OK the easy way if you don't know how too weld and identifying several types of welds that can appear very different on alloyed metals.
or its painted or specialty coated or covered in stickers the tap touch ring method works easy enough mild high carbon steel rings a dull ting thumping sound chrome moly rings clear and true its much denser and thinner tubing for lighter weight frames it rings much clearer and sharper than mild hicarb steel and aluminum is easy to ring test its very very dull and may not even ring with a dull bong dong sound
some magnesium alloys with aluminum can ring depending on how its made cast or extruded or spun all light weight alloys usually will not ring unless hardened or annealed with a heat treating process . just a dull ting at best so the handiest tool to use a finger nail or marble or a hardened nut from an axle to tap the tubing right in the center so no guessing by looking closely at a nice weld trying to guess what the hell that metal is........ if it rings clear and true chrome moly if it is rather dull ding ting not ring its high carbon steel or a mild steel if its 6061 aluminum thump knock bong dull to nothing sound that dissipates rapidly those are the ost common types of tubing used for bike frames ... good luck