If they voluntarily give you excuses that are detailed and scripted, then they probably are. He or she has most likely projected your questions, based on their indiscretion, and preemptively created a story to cover it all up. Listen and note any details which are unnecessarily significant to their story (names, times, exact locations, etc.) and keep a mental list. When they finish telling their story do they shake their head, toss up their hands, or use similar gestures to mimic exhaustion, confusion, and disbelief about their 'crazy evening' or event? Body language is a great indication that he or she is 'acting'. Keep it a seemingly harmless detail that won't frighten or alert your partner that you are suspicious (specifics are not what you are looking for).  Now ask them an abstract question about the specific detail you picked. Innocently ask when this 'detail' or event happened, or some other simple question. If he or she is not being truthful they will give it away during this question. Did they awkwardly get silent or shift before giving you an answer? Did they stutter or become nervous with their words? Did they look like a deer in the headlights? If so, here's why: When someone is telling the truth they don't need to think about the answer for more than a second or two. The details are already there because it truly happened. But, if this person is lying, they will have to stop and mentally go through their story from the beginning. He or she will need to remember where in the story this question happened and that takes thought processing. Since the 'minor' detail you asked them to provide wasn't prominent when they made up their excuse, they most likely won't have a cut and dry answer. Keep watching closely. Can you see them thinking it through again? Are they becoming frustrated and starting to snip at your 'dumb questions' and/or ask, "What does it matter?" If so, this is a stalling technique they use while making up new details. It's also a handy distraction if they point their finger back at...

TL;DR in Indonesian:
Dengarkan “cara” pacar Anda menjelaskan perilakunya. Perhatikan respons pacar Anda. Setelah hening beberapa menit dan cerita pacar Anda selesai, ingat-ingat satu detail yang menempel di kepala Anda. Perhatikan apa yang terjadi selanjutnya. Lakukan sekali lagi dengan pertanyaan yang mendetail. Bersiaplah untuk ditanya balik. Tunggu dengan sabar. Tanyakan diri sendiri, apakah Anda menyindir bahwa pacar menyembunyikan sesuatu?