The difference between strong (past participle with ‑en) and weak (past participle with ‑t) verbs is that the former constitute a relatively small, closed class whereas the number of the latter is increasing. This is because new verbs inflect weakly: gemanagt, gebloggt, gedopt, gebootet, getwittert.
The overwhelming majority of weak verbs inflect completely regularly; preterite and past participle are both formed with ‑t.
leben, lebte, gelebt
warten, wartete, gewartet
Two common irregular weak verbs are bringen and denken. These have a different stem in past forms.
denken, dachte, gedacht
bringen, brachte, gebracht
Then there are Rückumlautverben (Wikipedia) and modal verbs (Präteritopräsentien, Wikipedia), which show a different vowel.
nennen, nannte, genannt
müssen, mußte, gemußt
Strong verbs always have alternating vowels known as Ablaut (see Wikipedia). These must be memorised. The ending is -en in the past participle; the preterite has no ending.
sehen, sah, gesehen
kommen, kam, gekommen
finden, fand, gefunden
nehmen, nahm, genommen
schreiben, schrieb, geschrieben
There are lists of strong and irregular weak verbs all over the Internet, for instance on deutschlernerblog.de. What's great about that page is that there are several different lists for different CEFR levels. Wikipedia has an overview over the different kinds of irregular verbs. (They also have their own list of strong verbs, but grouping them into Ablautklassen is useful only for historical purposes and not for language learners.)
The best strategy would seem to be to memorise most strong verbs and then assume that every other verb is weak.
Note that derived verbs usually follow their base. So if you have correctly memorised weisen, wies, gewiesen, you also know how aufweisen, beweisen, hinweisen, verweisen, vorweisen etc. are formed. This is how the list linked above gets from 170 strong and irregular verbs to 1100.
weisen, wies, gewiesen
aufweisen, wies auf, aufgewiesen
beweisen, bewies, bewiesen
However, beware of homophones!
fehlen, fehlte, gefehlt (weak)
befehlen, befahl, befohlen (strong)