The first rule of thumb is to check on the eleventh house and its lord for such matters. This is in relation to ones spouse's tendency to flirt. While eleventh house stands for friends, gains in business, elder co-borns, income, gains, realization of hopes, ankles this is the house to know for spouse's lovers. Since eleventh house is the fifth from the seventh house. However, eleventh house would be indicative of spouse's love affairs depending on the weakness of the seventh house as well. There should be a relation between ones weak seventh house and the eleventh house. If ones seventh house is weak or the lord is ill placed and if the eleventh house contains an expansive planet like Guru or sukhra then we can be certain that during the period of elevent house the spouse will fall into relation outside of marriage. Because Guru / Sukra wants to have it all.
If you wonder about your spouse's chastity, check eleventh house, its lord both in rashi and navamsha. If you are undergoing the eleventh house lord dasa and if the current transit brings rahu ( foreigner) and if one goes through rahu ketu return. For the case of an example, a Makara lagna having mars debilitated in karka the seventh house could bring marital troubles due to the same but not necessarily the end of marriage.
For vrischika lagna, L-8/L-11 Budha is an especially strong indictor of extramarital affairs. Lord of randhra bhava is usually responsible for termination of the first marriage whether through death of the spouse or simply saturation/ death of the marriage i.e., divorce.
Simultaneously during a bhukti of of youthful, mentalized Budha, the Vrischikha native may experience the breakdown of fossilized marriage & the possible development of extramarital affair - but of course this is only likely if Budha's condition warrants such behavior.
Similarly Vrishaba lagna people should watch out for the periods of Guru dasa or bhukthi
