The OP has developed their own set of rules for how to IFF (Identify Friend or Foe) other Cmdrs. Regardless of what they then go on to do with that information (blocking in this case) that's a useful policy to adopt.
I have some fairly simple rules I use to figure out the threat level of another Cmdr I may happen to instance with:
Cmdr name. If they have a name that is 'cool' or especially 'intimidating', they are not a threat. Cmdr Dragonkiller (random example, apologies if that is actually a real Cmdr name), Cmdr Deathstalker. If they have a 'silly' or funny name, and particularly if they have a funny looking avatar is absolutely a potential threat, pay attention to them because they don't take the game seriously and will not be concerned about either your or their death. A Cmdr with the name of a fictional character will probably have a threat potential based on how cool or funny it is. A Cool name probably isn't a threat, a silly name is one to watch. There are lots of names that don't make threat potential obvious, normal (or what could be) names, single word names that aren't obviously cool or funny.
Ship type. An FDL, mamba, chieftain-type, dropship-type makes a poor trader, mission runner or explorer. A T-9, T-7 or T-6 doesn't make a great killer (a reason Screemonser's Pirate build T-7 is so awesome). A Corvette, T-10 or a conda probably isn't much of a PvP threat, most ships are faster but in a wing they are a much more likely threat. A lone cutter will be a trader but in a wing they could be a serious threat, try to locate their teammates. Other cutters will just be traders, combat ships will be a gank or pirate wing.
Body language. Is the ship headed towards you? Does it look like they are reacting to your presence? One thing I often do is o7 every Cmdr I see while not pointing my ship at them to indicate that I don't consider them to be a threat to me, and that I am not a threat to them. An o7 while pointed at them means I am watching them, either to indicate that they should consider me potentially hostile or that I can see what they are up to & am ready for it.
Ship Loadout. Very much one of the last things I look at, and only if I haven't already determined threat level from other info and still consider them a potential threat. I mean, I've never looked over the loadout of a T-9 for example, I don't care whether a T-9 pilot has an interdictor or frags or whatever, the Cmdr name & their body language will tell me everything I need to know. But an FDL might just be some novice that's read it's the ship to have, loadout will tell me. Any ship with a random collection of weird weapons, scanners & an ECM is almost certainly a newbie trying stuff to see what works. All lasers is a PvE combat ship, lots of frags or possibly lots of multi-s is someone that knows what they are doing, a couple of rails is high threat potential, all rails is probably a novice.
There are exceptions, these are all very general rules that I apply.
So what rules do you use? If you're a ganker how do you identify an easy target, If you're target focused PvE player what clues do you use to identify potentially hostile Cmdrs?