Basically just common sense.
If an offer looks too good to be true then it is especially if they offer any one of free accommodation, free flights or free food .
Easy to Google average salary for a job in Canada as many fake offers offer ridiculous compensation.
If they have a website generally these are cloned and they leave some data from the original site. On here for example on fake sites we see hotels in the middle of Canada with sea views, oil installations claiming to be in Canada but actually in Australia, So you get the idea.
Also you can check how long ago a web domain was created and if in last few months generally will be a scam.
Other indicators which maybe not so easy if a non native English speaker would be that the grammar of some offers tends to be pretty bad.
In general though job offers that appear to be from hotels, construction and oil/gas all tend to be fake
https://www.canada.ca/en/immigration-refugees-citizenship/services/protect-fraud/internet-email-telephone.html