With subs or any degree of BDSM, you assume nothing.
Talking through things with them as others have said, is the key.
If any supposed sub, gets impatient or brands you a timewaster, they are no loss. They are not really a sub. They may have the odd submissive tendency but a real sub appreciates and understands the need for discourse. They should welcome it.
Also, in my experience most truly sub girls seem to be very bright and perceptive indeed. They'll understand if you are serious, novice, experienced etc. the only problem is that of they have to 'train' a new Dom it can sometimes be quite laborious for them, but most will understand and do it.
Good luck!