If you can see bubbles, it is likely ML, assuming AF is complete. If you don't see bubbles, that does not necessarily mean it is complete. As mentioned previously, chromatography is my testing method and I know it to be qualitative and understand it's shortcomings. That said, should we care? For me. it's completion coincides with when I add SO2. What if I add too early? ML might restart if I don't keep SO2 high enough. That's not bad. If it does, either kill it or let it go. Many people in cooler climates report their wines restarting in the spring as their wine warms up. Waiting too long could risk some oxidation. However, if you practice good sanitation and keep the wine topped up with a minimal headspace and a bubbler, I don't think you would have a problem.
I MLF in my barrels with a little of the fine lees included (see: sur lie). The barrels are probably loaded with bacteria from an earlier wine but i still add a new dose of MLB every year. Because my barrels are 50L, I rehydrate the MLB and divide the liquid between my barrels. I stir the lees every couple of weeks and after I determine (chromatography) that MLF looks complete I switch from breathable bungs to solid. A few weeks later I add SO2. During this time, my barrels are full topped and I am not afraid of spoilage nor oxidization. I have had the bungs blow off a time or two which tells me MLF was not complete and my SO2 had dropped too low to prevent it from restarting.
All in all, my wines turn out very well with few exceptions -- mostly in spite my efforts and not because of them (this is my 19th vintage). Find good grapes, make minimal adjustments, let them ferment and, above all, keep the SO2 level up. It's fun to follow the process and build a knowledge base to react; just don't get caught up in the numbers. Tasting is more fun anyway.