Snowflake, most younger chickens show the paralysis. The older they get, the more likely they just show wasting, which is usually tumors taking over . I lost 5-6 that seemed to die with no reason before I realized that it was Mareks. The first ones died over a bit less than 2 years. Then I hatched 10 bought eggs under my silkies, and at 6 weeks, they got paralysis one by one about a week or less apart. At that time I was treating a hen for a "broken leg". Then one of those hatched chicks had a "broken leg" that I treated, when the third "broken leg" showed up, I knew what I had. All my deaths were on chickens under a year old, except 2-3 who were around 2 years old.
You can take a live ill bird over there, or a dead one that's been refridgerated up to a few days. On a younger chicken, they may find super small tumors or enlarged nerves, like the nerve in one leg will be alot bigger than the other leg nerve.
Skipskin, there is no way to tell if the neighbor's chickens carry Marek's. There's no test. My 20 or so original chickens, now 4-5 years old, are exposed, but haven't gotten symptoms or died. But they can still give it to other young chickens unless they are vaccinated at day 1 and quarantined a few weeks or more to give the vaccine time to work.