Iphone 6 screen replacement/logic board

Hello everyone,
First post in this forum so bear with me. 🙂
I have found an iphone 6 128gb that was handed to the police and then returned to me after 6 months as it was not reported lost or stolen.
I then decided to embark into trying to fix the problem.
The phone initially presented screen issues such as lines and blank parts that denoted either a faulty screen or cables. The sound and such was working and was recognized by itunes although the software was locked.
I then decided to order a new lcd-digitizer replacement (not an original one from Amazon) ans substituted. The new screen worked for a while although lines were presented on the screeen and then somehow the home button stopped working as well. I replaced the home button and the phone wa detected by itunes and I managed to get in to DFU mode and restore the software and I can access the content of the hard-drive and transfer files etc to it but still the screen is blank and not functioning.
I then decided to replace the ribbon-cables etc and the selfie camera modules etc. phone detected by itunes but no screen. I have purchased a new screen (original apple – spare from other phone) and waiting for it.
In the meantime I d like to figure out whether there is logic board damage.
How can I find that out as I do not want to waste anymore money to fix this.
The memory – hard drive module seems to be working so I doubt it’s the logic board but I’d like to rule that before spending anymore money.
Any ideas or suggestions would be really much appreciated