The fish on the 'business end' of your line and your rod bend will give you a reasonable approximation. Now if you toss in moving water/type of fish, must admit it will get far more interesting.
Most fish (moving water) will tend to try to pull swimming up/down stream and no real big deal. Ahhhhh, but "The exceptions are what proves the rule?" Kalama River in SW Washington.
Those Bug gers would lay flat across the current and actually 'cup' their bodies into the current. That 7# fish just became 14 until you could get his/her nose turned around. 'Only' thing you could do was swing the rod tip from as far up stream as you could get it to as far down as fast as possible to change line direction.
Doesn't sound like a big deal, but with a 13-14 foot 2-hander that was a line direction change direction of 26 to 28 foot. The 'idea' was to "tip the fish over" and that confused the heck out of the Fella. Or so you hoped ....
Actually, it usually worked ... assuming line wasn't still being ripped off your reel. But then you had a whole different set of problems that would, with little exception, end up as "A long range release."
But what the heck, you were going to turn the fish loose anyway.
fae