allievo

If the contact is just drawn on the nav map and you get NO message in radio room, the vessel must be a single merchant. On the nav map by clicking the drawn mark you can determine its speed:
stationary = 0 kt
slow: <6 kts
medium: <12 kts
fast: 12 kts<
very fast: aircraft
and its heading like North, Northeast etc. As you can see this information is merely an estimate but most of the times enough to work with. In stock game you can even know whether it's friendly, neutral or enemy by the colour of the marking on the nav map.
blue = friendly
green = neutral
red = enemy
In GWX the colours have been disabled so when receiving such a contact report, each drawing will be grey and you cannot know if the contact is a valid target thus you must identify her flag within visual range. At night most (but not all) neutral merchants are lit making it easier to distinguish them from enemy targets.

Contacts, that APPEAR on the message log, report the status of a convoy, task force or warship. These reports are more accurate than the ones without radio message logs i. e. they contain the target's exact speed in knots and heading in degrees. They also determine whether it's friendly, neutral or enemy. However if you get a report of e.g. an enemy convoy, there'll still be possibility you can find neutral vessels within that convoy usually in the rearward. This works vice versa i. e. enemy vessels in a neutral convoy are common, too.