1) Check who is the ultimate owner/who operates the app.
2) Check to see where they operate from, which will then direct you to checking with the appropriate regulatory body.
3) Check said company are regulated by and listed on said regulatory bodies website.
4) Check to see if there has ever been any finding against said company.
Theres probably more to it, but since they are regulated by the FCA which have very specific rules in the segregation of client / company assets, they should be relatively safe. Cash funds are protected up to 85k, and your investments are held in a segregated account with Interactive Broker.
That was my view before signing up.