Be careful of the discount for paying 100% upfront especially with stateside schools - if it don't work out you have a problem. The motivation to train you at some of the bad schools is to extract more money from you so if there is no more money to be extracted from you then they have lost their motivation!!
Once you have a shortlist of schools, try putting the name of the school in the search function of this forum, you will very soon see if the school is trustworthy. The extremely good and extremely bad schools have already been mostly identified by the people who have already gone through the school and posted on these forums. Sadly some only learn of PPRuNe after they have been through training so you are way in front of many of us!