Hi
Start at the front end of the circuit (V1 and V2) and work your way to the output (V7). At some point, you will find a tube with junk on the plate. Once you know which tube that is, you can start trying to figure out what the cause is. Equally, you can start from V7 and work back towards the front end. There are a couple of branches in the circuit, so it's not entirely a linear process. An alternative is to make a list of all the plate sections (V1A, V1B, V2A, V2B ....) and check each and every one of them. Check off which ones have junk and which ones do not have junk.
As always - be careful to keep things grounded and avoid touching the signal leads (High Voltage ....).
Bob