If you buy from any reputable seller they should list details on the type of flux and it's properties. If you really want solder without flux look at plumbing solder, although this tends to be quite thick.
I'm yet to see a flux with any good specification. I have like 15 fluxes, none of them have any important details, including amtech, etc. No flux specifies resistance, esp. at different temperatures. No flux specifies acidity when not activated (useful for hand soldering), etc.
So, the only real solution is clean it well (but may not work well for bga). Or characterize manually (that's what I did). I have only one flux that is claimed to be compatible with precision equipment, but again doesn't tell if its claimed resistance is unconditional, or after full activation.
Why not clean it? I found that good cleaning is quite difficult. Probably, an ultrasonic bath with full submerge is the only solution. Cleaning with a tooth brush is not efficient, imho. It's hard to clean clean under ICs, for example. It's not easy to clean solidified flux residue.