Modal auxiliary verbs:
can, could, may, might, should, would
Other modal verbs:
appears, look, seem, tend
Probability adj:
Likely, possible, probable, unlikely, supposed, presumed,
Probability adv:
perhaps, possibly, probably, presumably, arguably, in general, fairly, more or less, mostly, quite, somewhat
Frequency adv:
generally, usually, often, occasionally, seldom
Nouns:
appearance, indication, inference, likelihood, tendency, probability, possibility, to my knowledge
Verbs:
appear, assume, estimate, indicate, intend, propose, seen as, speculate, tend