Apex has a System class called Limits that lets you output debug messages for each governor limit. There are two versions of every method: the first returns the amount of the resource that has been used in the current context, while the second version contains the word limit and returns the total amount of the resource that is available for that context.
The following example shows how to embed these types of statements in your code and ultimately determine if or when you are about to exceed any governor limits. Using either the System Log or Debug Logs, you can evaluate the output to see how the specific code is performing against the governor limits. Additionally, you can embed logic in the Apex code directly to throw error messages before reaching a governor limit. The code sample below has an IF statement to evaluate if the trigger is about to update too many Opportunities.
Here is an example of how you can use a combination of System.debug statements and the Limits Apex class to generate some very useful output as it relates to governor limits and the overall efficiency of your code.

trigger accountLimitExample on Account (after delete, after insert, after update) {

    System.debug('Total Number of SOQL Queries allowed in this Apex code context: ' +  Limits.getLimitQueries());
    System.debug('Total Number of records that can be queried  in this Apex code context: ' +  Limits.getLimitDmlRows());
    System.debug('Total Number of DML statements allowed in this Apex code context: ' +  Limits.getLimitDmlStatements() );
    System.debug('Total Number of CPU usage time (in ms) allowed in this Apex code context: ' +  Limits.getLimitCpuTime());
    
   // Query the Opportunity object 
    List<Opportunity> opptys = 
        [select id, description, name, accountid,  closedate, stagename from Opportunity where accountId IN: Trigger.newMap.keySet()];
   
    System.debug('1. Number of Queries used in this Apex code so far: ' + Limits.getQueries());
    System.debug('2. Number of rows queried in this Apex code so far: ' + Limits.getDmlRows());
    System.debug('3. Number of DML statements used so far: ' +  Limits.getDmlStatements());    
    System.debug('4. Amount of CPU time (in ms) used so far: ' + Limits.getCpuTime());
    
    //NOTE:Proactively determine if there are too many Opportunities to update and avoid governor limits
    if (opptys.size() + Limits.getDMLRows() > Limits.getLimitDMLRows()) {
            System.debug('Need to stop processing to avoid hitting a governor limit. Too many related Opportunities to update in this trigger');
            System.debug('Trying to update ' + opptys.size() + ' opportunities but governor limits will only allow ' + Limits.getLimitDMLRows());
            for (Account a : Trigger.new) {
                a.addError('You are attempting to update the addresses of too many accounts at once. Please try again with fewer accounts.');
            }
    }
    
    else{
        System.debug('Continue processing. Not going to hit DML governor limits');
        System.debug('Going to update ' + opptys.size() + ' opportunities and governor limits will allow ' + Limits.getLimitDMLRows());
        for(Account a : Trigger.new){
            System.debug('Number of DML statements used so far: ' +  Limits.getDmlStatements());
            
            
            for(Opportunity o: opptys){ 
                if (o.accountid == a.id)
                   o.description = 'testing';
            }
           
        }
        update opptys;
        System.debug('Final number of DML statements used so far: ' +  Limits.getDmlStatements());
        System.debug('Final heap size: ' +  Limits.getHeapSize());
    }
}