Yes, a set sell target is easiest for sure, but if you are able...

Yes, a set sell target is easiest for sure, but if you are able to watch for a bar or two it may help (or not...haha).

You can never be 100% sure just how strong potential resistance will be, however there are a couple of things that may help.

Firstly, it is only the trading to the left which has not seen price action recently, that should be considered when attempting to decide if the expected resistance will be strong or weak.
If you look across to the left, at the price action (or congestion) that has caused you to draw the potential resistance line/zone in the first place. If that congestion contains higher volume, there is a higher potential for it to contain stronger resistance (generally the higher the volume, the stronger the potential resistance...and vice versa).
Also the longer price spent building that congestion (the wider or broader it is), the stronger the potential resistance maybe.
Also the further away in time and price, that potential resistance has been.....should reduce the strength of potential resistance to some degree.
Finally, if price has a really really serious climax behind it, and/or a very large accumulation zone, that should also reduce the strength of potential resistance somewhat.

Remember that resistance will be caused by supply coming in (aka sellers), and that supply will come from either - stale holders who are holding (at a loss), at the level they bought at, with every intention of 'getting out' as soon as price comes back to their level again.....or profit takers from positions opened below the current price........and/or new short positions....... (or obviously, some combination of the three).

So there is no perfect way to exit a position.
Steve Phillips would say that you should have a trading plan, and within that overall plan, there should be various planned exits, depending on what has actually happened (winner, loser, scratch). If you do this, then no matter what happens, you do not need to stop and think, as all the different options have already been considered in the trading plan, and all you have to do it execute depending on what the price action has done.

That said, there are a few different easy strategies that can be considered (whether you have a written trading plan or not).

Firstly, just take profit at a predetermined target, which will usually be at the next higher potential resistance zone (and should be set when the position is opened....before greed sets in). This way you have done what was anticipated, you get out, and look for the next trade......done.
Or a variation of that is to to sell only a proportion at this point (usually half, or capital & brokerage back - free carry, but it can be any amount you like) when price has reached the initial target, or the potential resistance zone above, then use a crawler trade to trail the remainder of the position up with a (fairly wide trailing stop), until you are stopped out.

The crawler trade is a strategy all by itself. Which is a good one for trending, or Wyckoff style, trades. The idea behind it is to stay in a trending trade for longer, and not to take profit too early.
So the idea is to have a fairly wide hard stop, which is lifted up each time a new minor low pivot has been put in, and price has broken out above it again (there are a also few variations, which are to keep the stop a bit closer by lifting the stop to just below the previous bar's low, or the two bar low, three bar low etc.), and you don't ever take profits per se, you just keep lifting the stop until price comes back enough to stop you out.
This strategy should keep you in a trending position for much longer, especially if you consider a stock may just 'keep going' for an undefined time (and you don't want to miss out....ha).
Anyway, the downside is that some potential profit will always be lost (depending on how wide the stop is at the time), when using this strategy, because at some point price will be much higher, before coming back and stopping you out. And the difference between the highest point price has traded at, and the stop out point, is lost profit.
I have heard of some traders who start off with a really wide stop (using the minor pivot position, and then to protect profits, switch to something like a three bar low, then a two bar low, slowly tightening it up until you are stopped out. Which can be a good if you are holding a nice profit (sadly if it just keeps going higher after stopping you out, you will still miss out on potential profit....but no strategy is ever perfect).

There are numerous other different trading strategies, some can be very rigid and mechanical, and others more flexible, depending on the position, and particularly the market you are trading in.

Hope that helps a bit.

cheers