Haulmark of Quality
Understand your trailers weight rating. Some manufacturers tell you how much the trailer and load can weigh, others rate only the load so two trailers rated the same may actually be rated for different load capacity.
Ask if the company has engineering supporting the trailer weight rating
Axle quality varies significantly. Stick with a name brand and check parts availability. Dexter has a good rep.
Examine the welds carefully. If you don't know welding, get a welder to look at trailer construction with you. A trailer builder near me uses after school high school boys to weld their trailers. Quality control of welding is typically better with name brand products.
Ask about the quality of steel, obviously a trailer of T-1 or 130 steel would be harder and stronger than a trailer made of lesser steel. How quickly the frame edges get torn up from chains demonstrates the greater wear factor of lower quality steels. They also bend and break easier.
Happy Trails