The biggest part is as Carvs mentioned, that if a verb does not have -iru or -eru at the end, it will 100% be a -u verb.
So, わかる must be a -u verb because it ends in -aru.
However there are also some -u verbs that end in -eru or -iru: for example 帰る, or 限る.
But there’s another hint to see whether an -iru / -eru verb conjugates as a -u or -ru verb: the okurigana. If you compare
帰る (-u verb)
変える (-ru verb)
混ぜる (-ru verb)
見える (-ru verb)
限る (-u verb)
走る (-u verb)
握る (-u verb)
借りる (-ru verb)
浴びる (-ru verb)
降りる (-ru verb)
You might notice that the -ru verbs have the -i / -e hiragana behind the kanji as okurigana, while the -u verbs only have る directly after the kanji.
With this, you can determine for another good portion of verbs (at least when written in kanji) whether they are -u or -ru verbs.
The only problem remaining are verbs with only two morae:
知る, 要る, 減る, 見る, 切る, 着る
I’m afraid you will have to memorize these.
(I am only aware of one exception: 混じる・交じる, but there might be more, so no guarantees or anything.)