Light Check
The time-honored way to check your trailer lights is with an assistant who embarrasses you by shouting which lights are working as you go through the checklist: running, brake, right turn, left turn, four-way emergency flasher.

No helper? At night or during the low-light periods of dawn or dusk, drive to the nearest parking lot with spaces near big windows. Back carefully into a space in front of a window, and watch the reflections to see that all lights are working.

Towing Capacity, In a Nutshell
The manufacturer determines the towing capacity of a vehicle. It's based on factors like brakes, suspension, axle ratios, engine power, engine cooling system, transmission type, transmission cooling system, electrical system, etc. A lot goes into it. Don't be misled by advertisements. A vehicle may be advertised with massive towing capacity, but if you select a slightly different version of it — optional cab design, engine size, etc. — you could greatly impact the capacity. Check the owner's manual and, if necessary, with the manufacturer before towing a boat, and before buying one.