It really depends on the kind of details you want. Since you specifically asked for technologies we know its at least theoretically working, I want to focus on what we know from experience that works, followed by a wild speculation.
If you know the position of the suspicious vessel, you can use either radar or some kind of lidar (basically a laser-radar, using laser-frequency photons instead of radio photons). Both will be able to determine the current vector relative to your vessel.
Current velocity however isn't very indicative of what you want to know. But fortunately, both radar and lidar should be able to determine the spacial form of your target, specifically what kind of propulsion is used and how large it is. You might calculate an approximate maximum accelleration based on this.
What I think might be even more indicative than the exact dimensions of the drive itself would be the means of heat dissipation. Power systems produce a lot of emissions, especially with fusion-powered systems. You need to get rid of that stuff, so look for heat sinks.
The vessel will need as much heat sinks as it has power available, otherwise it will fry itself when going full power.
You could use the reflection behaviour of the material you're looking at to determine it's type and guess it's use, not every surface needs to be a heat sink (see: story weakpoint)
But there is a caveat: unless you have drones in the proximity of your target, you'll probably not be able to see every heatsink, as you won't get any reflection from the sides not facing your own vessel.
Also, the range might be limited, because after bouncing from your target, the radiation will scatter much more, and radiation will, unless it is lased, diminish proportional to the second power of the distance (see: inverse-square-law).
Thus, you will probably close in distance much more to have sufficient sensor resolution.
Now this is a weak point of your technology stack: Technically, it isn't too feasable to use fusion power, as the only ways to get rid of the heat in space are either dropping coolant or blackbody radiation. You could estimate needed radiator area by using Getting rid of heat SE Unless you want to overlook this scientific fact in your story (it is science-fiction after all). You could always invent some way to address this.
Other things to consider:
You probably wont find anything that is penetrating the hull of the suspicious vessel, as it will be heavily shielded against everything freely floating in space, like microwaves, gamma rays etc., since they will be facing the sun at some point (probably most of the time). Even if you could, you'll be killing the crew anyway.
It seems to me, that in many cases, it doesn't necessarily depend on whether you use active or passive sensors as much as your distance and the targets emissions. These, in many cases, have a much higher factor in determining detail than the type of sensor you use.