I don't know if Tor is still following this thread, but if so:
As Alpo suggested, throw some sticks. I'd put some refinements on that suggestion: You want the stick to be bright, so a few 10 cm bits of "flagging tape" - fluorescent orange plastic tape - tied around the stick would help visualize it. You want to see how that stick behaves in the worst flow path - which is towards the center of the outlet, but bottom conditions could cause it to be to the left or right of center.
How to throw sticks? A "wrist rocket" is used by 10 year old boys to kill small birds and "put an eye out" of a playmate. My local hardware store has them on sale for $4 every other month. A length of rubber tubing (3 or 4 meters) tied to two trees would give you MUCH more energy for your launch (or are you north of treeline?).
Or, instead of sticks, "range balls" - golf balls rented by the bucket to drive into a lake at the golf course FLOAT (that's how they retrieve them). Some are white. Some are fluorescent orange. All could be shot from a wrist rocket, rubber tubing between two trees, or from a Jai alai pelota (racket) or those long atlal-atlal type handle for throwing tennis balls for your dog to retrieve.
I think your original post wanted an informed opinion like, "water velocity will be less than 1 kph if you are 3.5 outlet-widths upstream of the lake outlet". I've studied and use a fair but of fluid dynamics and hydrology in my work and I can assure you that no such simple rule of thumb exists. If you show me a non-topographic map, I'll know nothing about water flow near the outlet. If you show me a map with topo lines for the land, I'll be guessing about the shape of the lake bottom. Only if you have bathymetric and river flow data for the time in question could I offer any estimate of how close to the outlet does the flow speed up. Broadly, the deeper the water you are in, the slower the flow. I can think of glacial lakes (I live in a very glacial area) that have a long, shallow area near the outlet (BAD for a swimmer). And I can think of other glacial lakes, typically with a terminal moraine at their outlet, that are very deep until very close to the outlet (not much of a detour for a swimmer).
Floating sticks would let you assess that on site.
Bathymetric maps would let you assess it in advance. PM me a link to a map if you want an opinion.
Best of all would be a companion who doesn't swim as fast as you do. If he gets sucked into the lake outlet and crushed on the rocks below, then you need to take a longer, more upstream path.