It's all about knowing the role!
All military TPs are selected after extensive military operational experience on the front line and many also have instructor qualifications. This means that they can work within a test team to provide credible role relation when determining whether a found deficiency is actually an issue in the role, or not. Most large helicopter manufacturers build a lot of military helicopters and don't have many TPs, they therefore usually go for ex military TPs. That said, extensive offshore experience and a civvy TP school qualification may work (expensive though!). EASA now have 2 classes of TP; Class A (experimental) and Class B (developmental) you will need to make sure any course you do is accredited or has grandfather rights.