As a general rule of thumb, watch the way your tank pulls. You'll see that the mobs' names are different colors depending on whether the tank has managed to get threat on them yet. Unclaimed mobs that are not yet on the attack will have yellow names. Once the first mob is grabbed, you'll see the name on that mob turn red. The other mobs that attack at the same time that one is pull will NOT turn red, but will darken slightly to a lightish orange color. Those are the mobs that are going to come gunning for you if you do any curing (including Regen).

A good tank will try to turn every mob red asap as they pull, but less experienced tanks may pull multiple packs without cementing hate on each group first. Those are the ones to watch out for, and hold back the regen.

Note that even for good tanks, there are cases where cementing hate right away isn't always practical. In Amdapor Keep, for example, high-level tanks like to pull a ton of mobs at once, since they are very weak compared to other level 50 dungeon mobs. At certain points in the dungeon, there are statues that animate when the tank gets close. These statues are spread out, so running to each and flashing takes quite a bit of time; many tanks, even experienced ones, will simply trigger them and keep running.

Lastly, if you DO get regen hate (or any kind of hate, really), the best thing to do is run right up to the tank. This will allow the tank to quickly pull the hate off of your with a Flash/Overpower/Unleash or two. Don't run away, or around in circles, as that just makes it harder for the tank to pull them off you - and it stops you from healing yourself and others at a time when it may be crucial.