How to check participants in at events?
CiviLearner,

You will need to use Find Participant search form to update the status of participant. 

Do a search on event name= [name], role = 'attendee' filters, select participant and then from Actions list select 'Update multiple participant'

Or You can use Advance search with same filter but select 'Event Participants' from the 'Display Results As'.

HTH

Pradeep