It is a little oddity of English, but while police is clearly singular, we treat it as plural, so the verb would be "are" here.
I have. I sometimes put a comment here as well, just in case there is someone whose first language is not English.
I do not know what original question was, but, here is the result of my research. Policja is Nominative singular it has no plural, szuka is a verb in gen. that the direct object of the verb is always gen. so, dowod becomes dowodow (accent over O should be there) which is plural in gen. The police are looking for all evidence not just 1. RPS
Just note that „Policja” has normal plural „Policje” – it's just rarely used, because „Policja” alone means the whole of Police force(in it's primary meaning), so it's only used when you mean Police forces of multiple countries:
„Przestępczość w rejonie przygranicznym nadal przekracza średnią całej Brandenburgii. Aby temu przeciwdziałałać, policje Polski i Niemiec zacieśnią współpracę - podaje portal niemieckiej rozgłośni RBB.”
I get the idea that the Poles treat policja as a singular, so szuka would be right.
Could someone please elaborate and explain the uses of 'dowody' and 'dowodów', such as how to use which and if it is always used in plural form?
I think it's just a presumption that there probably should be more than just one piece of evidence. Technically, the police probably could search for just one, but would that be enough? So if it's such a general sentence, we'll rather use the plural.
"Dowody" is either Nominative (To są dowody = These are pieces of evidence = This is evidence), or Accusative (Widzę dowody = I see pieces of evidence = I see evidence). "Dowodów" is Genitive, and this what is needed here, after 'szukać'.
This is just an English error here. I typed "The police is looking for an evidence" and got it wrong. The correction told me "a evidence," and that's not correct.
Corrected it, thanks.
Although frankly I'm not sure if we should accept a singular "an evidence" if Polish uses plural and English usually just "evidence", but let's leave it aside.
Evidence is an uncountable noun; therefore, this is fine. "Evidences" sounds very weird.