How can one be assured that the Da'wah she is making does not contain any "Riyaa" its a disturbing thing and sometimes I get a feeling of hesitant to do the Da'wah even though with Allah's Grace, the ladies are very happy and anxious to get more teachings from me.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is a good and positive sign of one's faith to accuse oneself of negligence for fear of al-Riyaa (a form of Hypocrisy /showing off one's actions). Such a stand also may lead one to hold oneself responsible for actions, to become aware of one's need for Allah and to be humble before Him.
One who thinks that he/she is all right and thinks highly of himself may lose such forms of 'Ibadah (worship), i.e. submission to Allah's Will, fearing al-Riyaa, holding oneself responsible, etc. But, fearing al-Riyaa should not prevent one from doing a good deed. Thus, our scholars say: working simply to please others is a form of Shirk (polytheism), neglecting such a good work for people constitutes al-Riyaa, and the best state is to achieve sincerity to be far from the two previous states.
So, one who is sincere and works for Allah's sake will proceed, push away every form of devilish insinuation, fear Allah, and seek His help to avoid al-Riyaa and achieve sincerity. In fact, such a state of struggle is a form of 'Ibadah which Allah will reward.
Furthermore, he/she should do his/her best to have such a state forever.
In brief, we advise our sister, to continue in her Da'wah activities for Allah's Sake, since they are of the best actions, and then, to turn a shoulder to devilish insinuations to distract you from the Da'wah on the plea of fearing al-Riyaa.
We, Insha Allah, hope that the others' love to you and their engagement in your work will be signs of Allah's love to you. So, continue in your work, and may Allah grant all of us success to do what Pleases Him.
Allah knows best.