I have a 20 inch BMX bike. It was given to me with no stickers or identifying marks on it. How do I tell the difference between brands?
You can try to find a serial number engraved somewhere on the bike. For road bike it is sometime under the bottom bracket, so you can try to look there first. Manufacturers might have different serial number pattern so you can start your search there.
Other than that, posting a picture on a forum or here might be a good idea since some people might recognize which bike it is.
Basically you can't tell the difference between most BMX "brands" as all of them made in china (exclude of aluminum BMX that are pretty expensive and therefor pretty rare).
The major difference in BMX bikes is quality of their bearings, spokes etc. - things that you can't see.
The only thing you can check (and it is very important to do) is the assembly of the bike. The pedal axle and the fork must be tightened well. If there is play in them, it says that it's assembly wasn't professional (or it was ridden much, if the bike is old). To save the bike in good condition for long time, it's have to be set up.