Performance and consistency
The only real way to properly test consistency and performance is out in the field. If you get a chance to test a paintball gun in open play then keep these factors in mind and make mental notes as you go along.
Rate of fire: how quickly does the paintball gun fire and reload?
Precision: to what extent does the paintball gun consistently hit a target when under a steady aim at that target?
Range: how far can the paintball gun shoot a paintball? Does the range fluctuate? If it does, this suggests that the barrel velocity (the chrono speed) is not consistent.
Reliability: will the paintball gun perform well all the time, or is it one of those infuriating cheap paintball guns that only work well on some days, or with a certain brand of paint? Try the paintball gun out will different types of paint and see if it has a prejudice.
Noise: how noisy is the paintball gun in operation? Is the noise so distinctive as to easily give away your position?
You must also ask other players for their opinion on the performance and reliability of a particular paintball gun. You'll find that real problems surface time and time again, so ask as many players as you can.
Durability and construction
Cheap paintball guns get as much abuse in the field as do expensive ones, but the expensive ones are generally built to last longer. Even so, a good cheap paintball gun will have to stand up to some serious paintballing action. Again, a field test is the only way you can really assess durability. Only after you have dropped it, rolled over with it, got it dirty, played all weekend and given it a really good pounding will you know. Even then it may be holding back some surprises for you. It is therefore essential that you seek comments from other players about the gun's durability. Do a search in Google for 'paintball forums'--with the internet you can speak to paintballers all over the world.
Having said this, there are things you can pick up on in the paintball gun shop. A good cheap paintball gun will have a good sturdy feel. It will be made of durable materials and will be well put together (build quality). As a quick guide to build quality, give it a really good shake. If it rattles, take a pass.
When buying a cheap paintball gun its best to avoid guns with a plastic body and grip, unless the plastic is of the exceptionally tuff polymer type. If you are offered plastic find out what type it is. Good cheap paintball guns generally have a metal body and grip. However, it is not unknown for bolts to come lose from metal bodies, or for metal body paintball guns to suffer from poor build quality. Again, the shake test will tell you something.
A one year warranty is the industry standard for paintball guns. You should also check out the manufacturer's website and see what the after sales service is like. Is there a number you can call for product advice? How quickly can spare parts be delivered?
In summary, consistency, performance, construction and durability are crucial qualities that all good cheap paintball guns will possess. It is possible for you to assess these but you will need the experience of others to make you assessment. Always try to test a paintball gun in the field before you buy.