The age a gilt has her first season varies a little with the breed but as a guide anywhere between 6-8 m .Some gilts get a little grumpy; their back end will swell a bit and become redder [not always easy to see the first few times] If in season you can push down on their hip area and they should stand still as if waiting for the boar to mount them.But young females can be a bit irregular in the beginning. Generally we put a gilt to the boar at 12m and just let them run together and let nature take it's course;the actual event can be rather noisy at times the gilt can often give a high pitch squel when being served the first time. If you're not about to witness the event the gilt will have a white mucousy plug for 12-24 hours after the event .
Hope that helps.