To answer your questions as to how you learn your market and when you know if it is changing, you will have to basically work numbers on deals and get a feel for the market while keeping your finger on the pulse as to how things are trending.  

Depends on the shape of the property but I've heard anywhere from 5-10% for estimating cap ex.

some obvious, some not so obvious...http://www.realtor.com/news/trends/noise-discounts/?

Here are the details:Duplex purchased in August, 2015 for $155,000ishLived in one side of the duplex until September 2016Both sides rented for $1,200/mo ($600/each) when I first bought itBoth sides now rent for $1,800/mo ($1,000 for renovated side/$800 for other)Cash flows well for me (about $8,000/yr) and I have a very reliable, cheap property manager (good buddy of mine); total expenses are around $1,125/moHaven’t added up all the numbers, but estimated renovation costs were about $25,000Reasons I am considering selling:Live in Dallas - don't want to deal with property out of townInterest Rates rising (slightly afraid of a market dip): my property manager will be leaving College Station in 2 years (once he graduates) and I might have trouble selling the place at that time if market is in bad shape; don't want to pay 12% for a conventional manager to handle it, but I could do that if need beConsidering buying and building in Dallas 'M streets' area (for primary residence, not investment) - which will be quite expensive; we have savings, but if we had to pay cash for a distressed home or lot in the area, then that would be tough without this capitalLooking to sell for about $230,000, which would be about a $50,000 profit:  don’t want to pay taxes on the gains, but I am unsure of how to manage the situationCan’t live in it for another year (to get to 2 of 5 years primary residence)1031 exchange to personal residence seems tricky – I assume I would have to exchange for another investment property, establish it as such, and then convert it to my personal residence sometime down the road; also the timing on these things seems absurd (45 days/180 days) unless you have something totally lined up, which I guess is the pointAm I up the creek and I should just sell it and pay the taxes?

Here goes: It is a Triplex built in 1950, I haven’t had the chance yet to visit it but it seems in decent shape.

She told me that the rental house next door is in such bad shape that it's hurting the value of my house. 

What I always do is check the real estate trends in the area I'm interested in before I make any investments.

He has done a great job in keeping these properties in good shape.

I ADVISED HIM I WOULD WORK WITH HER AS LONG AS SHE LEFT THE HOUSE IN GOOD SHAPE AND DID NOT TEAR ANYTHING UP ON THE WAY OUT.

I would love to connect with some other investors in the area to talk about market trends.