How long has the lash technician been applying the lashes?
Ideally you want someone who has got at least 6 months experience.  To apply the lashes well, takes a lot of hand/eye co-ordination.  Not something that happens overnight.

Have they got photos of THEIR OWN WORK to show you?
This is most important.  Most lash technicians will have on show lots of glossy pictures from various companies showing beautiful long lashes.  This does not mean the person you are talking to can do this for you.

How many lashes will they put on for the price?
The amount of lashes someone will apply usually varies from 30 to 150.  Its a massive difference and the result is obviously completely different.  Also some technicians use the thickest lash possible to get maximum result with the least work with no regard to the health of your natural lash.

Do they separate each lash and apply a lash to it?
This is a good test of your lash technicians skill.  If you can comb through them right from the base without even a tiny lash being caught in with another, even at the base where the tiny lashes are, that is what your'e looking for.  If you have even two lashes caught together, one will grow at a different rate to the other and start pullling on the slower growing one.  It HURTS, its sore and can pull out the slower growing lash.

Do they apply separate lashes underneath your own lashes?
Some companies that apply individual lashes do just that, but instead of attaching each one to one of your individual lashes they just place them underneath your own lashes at regular intervals.  This is going to be uncomfortable as they grow, you won't be able to comb them through and long term not healthy for your own lashes.  However, for a quick cheap look that you have professionally removed after about a week, perhaps they are an option for you. 

What type of lashes do they apply ie what are they made of?
There are synthetic (polyester), silk and mink mainly on offer.  All are good when used correctly.  Be wary of cheap, thick, hard plastic lashes, usually chinese imports.  A good lash will be very light, springy, flexible, thicker at the base, tapering to a fine point.  There is some thought that people with sensitive eyes might be more likely to react to the mink lashes.